The only advantage I can see Evio Clamperl having that Evio Omanyte does not is that it doesn't get destroyed by random HP Grass moves but that really is it.
See what Simbo said:
However, the weakness to fighting and ground is pretty huge for omanyte imo. Firstly, it prevents set up against the most commonly used mon in lc in mienfoo.
236 Atk Mienfoo High Jump Kick vs. 76 HP / 0 Def Eviolite Omanyte: 20-26 (95.2 - 123.8%) -- 75% chance to OHKO
Possible damage amounts: (20, 20, 20, 20, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 26)
Bulky foo is pretty common rn but even so
0 Atk Mienfoo High Jump Kick vs. 76 HP / 0 Def Eviolite Omanyte: 18-24 (85.7 - 114.2%) -- 6.3% chance to OHKO
Possible damage amounts: (18, 18, 18, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 24)
After rocks, omanyte is dead or has a high chance to be dead. Evio clamperl comfortably sets up on the no1 most used mon in lc, which is already great but being able to set up on every fighting type in lc barring iron fist timburr and gunk is something evio clamperl can boast but not omanyte.
Considering how common scarf foo + timburr fightspam teams are, clamperl's ability to (barring max rolls) take a regen scarffoo hjk, set up, then take the iron fist mach at -1 is super solid.
The weakness to ground results in omanyte being unable to set up on drilbur, mudbray and diglett. Clamperl reliably sets up on first two and can potentially set up on dig, depending on set.
The most important point here is the Fighting weakness. Being unable to sweep simply because the opponent has a Timburr or Croagunk is very troublesome.
Evio Clamperl misses a lot of crucial +2 KOs that Omanyte gets such as Mienfoo, Foongus, Grimer-A, etc.
+2 200+ SpA Omanyte Surf vs. 156 HP / 196 SpD Eviolite Mienfoo: 18-22 (78.2 - 95.6%) -- guaranteed 2HKO
+2 252 SpA Clamperl Surf vs. 156 HP / 196 SpD Eviolite Mienfoo: 16-19 (69.5 - 82.6%) -- guaranteed 2HKO
In my experience almost all Omanyte run Surf because, requiring quite a lot of support, it tends to be the primary wincon of its team, and so the consistency that Surf has over Hydro Pump is more valuable. Neither Omanyte nor Clamperl can OHKO slow Mienfoo with a +2 Surf. Admittedly though, Omanyte can OHKO slow Mienfoo after Stealth Rock with Hydro Pump, and Omanyte can OHKO the occasional fast Mienfoo with Surf, while Clamperl can do neither.
Unlike Omanyte, Clamperl cannot OHKO Foongus with a +2 Ice Beam, however, it can after Stealth Rock, so it's not a very big issue.
+2 248 SpA Clamperl Ice Beam vs. 124 HP / 160 SpD Eviolite Foongus: 22-26 (88 - 104%) -- guaranteed OHKO after Stealth Rock
As for Alolan Grimer, Omanyte has a 75% chance to OHKO 12 Special Defense Alolan Grimer with Surf only when Stealth Rock is on the field. That's very circumstancial as many Alolan Grimer run 14 Special Defense. You cannot rely on Surf Omanyte to OHKO Alolan Grimer because of the presence of the more specially defensive set, and so you need to weaken it before setting up anyway, at which point Clamperl would probably also KO. However, you are correct in that Hydro Pump Omanyte has a 75% chance to OHKO 14 Special Defense Omanyte after Stealth Rock while Clamperl doesn't, although, as I said previously, most Omanyte run Surf, so I don't think this is very significant.
Furthermore, all three of these Pokemon that you mentioned can have their Eviolites removed very easily, with Mienfoo and Alolan Grimer resisting Knock Off and Foongus checking Fighting-types. For these reasons I think that the slightly lower Special Attack of Clamperl isn't quite as big of a deal as you think. There are very few Pokemon that are OHKOed by Omanyte but not Clamperl that are not easily hit by Knock Off.
Also Clamperl doesn't set up consistently; it does the opposite in fact, because of how it has no secondary typing and its Defense stat isn't that great, especially compared to other SS sweepers.
Shell Smash Omanyte's secondary typing hurts it overall. The Fighting weakness is terrible and as Simbo pointed out, its Flying resistance is not important:
You mentioned omanytes rock typing providing a nice resistance to flying types but, considering most flying types have ways to either flat out ohko omanyte or get hefty chip massively reduces its set up opportunities on the flying types it's supposed to resist. I made a quick list comparing omanyte and clamperl setting up ability on the common and viable flying types in the tier and, as shown by the list, even with omanytes flying resist, evio clamperl still sets up more consistently against the flying types.
Vullaby:
Both clamperl and omanyte can set up on physical vullaby sets. They take the exact same amount of damage from ko so they set up equally well on physical vullaby.
252 Atk Vullaby Knock Off (97.5 BP) vs. 0 HP / 76 Def Eviolite Clamperl: 7-9 (35 - 45%) -- guaranteed 3HKO
Possible damage amounts: (7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 9)
Clamperl takes more from BB but it still sets up comfortably on vull and its less susceptible to prio if evio is not knocked. The only time omanyte really comes out on phys vull vs clamperl is if it sets up on scarf vull locked into bb.
252 Atk Vullaby Brave Bird vs. 0 HP / 76 Def Eviolite Clamperl: 9-12 (45 - 60%) -- 68.4% chance to 2HKO
Possible damage amounts: (9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 12)
Omanyte cannot set up on special vull cause:
240+ SpA Vullaby Hidden Power Grass vs. 76 HP / 0 SpD Eviolite Omanyte: 20-24 (95.2 - 114.2%) -- guaranteed OHKO after Stealth Rock
Possible damage amounts: (20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 24)
However, as highlighted in my previous post, clamperl easily sets up on special vull and can even set up on +2 vull at full.
Against vullaby, evio clamperl is far more consistent as an SS sweeper, setting up on more sets comfortably.
Doduo:
Scarf dod beats both omanyte and clamperl, since it outspeeds and revenges.
Bj dod beats omanyte if sub. Sub on the SS then:
236 Atk Doduo Jump Kick vs. -1 76 HP / 0 Def Eviolite Omanyte: 18-22 (85.7 - 104.7%) -- 6.3% chance to OHKO
Possible damage amounts: (18, 18, 18, 18, 18, 18, 20, 20, 20, 20, 20, 20, 20, 20, 20, 22)
236 Atk Doduo Quick Attack vs. -1 76 HP / 0 Def Eviolite Omanyte: 3-3 (14.2 - 14.2%) -- guaranteed 7HKO
Possible damage amounts: (3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3)
It also beats clamperl too.
Non sub bj dod doesn't beat either omanyte or clamperl. Both can set up on it.
Clamperl:
236 Atk Doduo Brave Bird vs. 0 HP / 76 Def Eviolite Clamperl: 10-13 (50 - 65%) -- guaranteed 2HKO
Possible damage amounts: (10, 10, 10, 10, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13)
236 Atk Doduo Quick Attack vs. -1 0 HP / 76 Def Eviolite Clamperl: 6-7 (30 - 35%) -- 1.1% chance to 3HKO
Possible damage amounts: (6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7)
Omanyte:
236 Atk Doduo Jump Kick vs. 76 HP / 0 Def Eviolite Omanyte: 12-16 (57.1 - 76.1%) -- guaranteed 2HKO
Possible damage amounts: (12, 12, 12, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 16)
236 Atk Doduo Quick Attack vs. -1 76 HP / 0 Def Eviolite Omanyte: 3-3 (14.2 - 14.2%) -- guaranteed 7HKO
Possible damage amounts: (3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3)
Assuming the most likely rolls occur, omanyte ends up with 1hp more after set up and dead dod, so they set up pretty equally against this set.
Both mons get bopped by lo dod, neither can set up on it.
Against dod, they set up equally well against one set, get bopped by the other two. Again, flying resist isn't particularly useful when compared to evio clamperl.
Rufflet:
Omanyte is likely to die to superpower without rocks and guaranteed death after rocks so you can't set up on any non-scarf set.
252 Atk Hustle Rufflet Superpower vs. 76 HP / 0 Def Eviolite Omanyte: 20-24 (95.2 - 114.2%) -- 56.3% chance to OHKO
Possible damage amounts: (20, 20, 20, 20, 20, 20, 20, 22, 22, 22, 22, 22, 22, 22, 22, 24)
Clamperl takes any one hit from rufflet, unless its z move. Therefore, you set up on any non-z move set or lo (idk if lo is a set, never seen it personally)
252 Atk Hustle Rufflet Brave Bird vs. 0 HP / 76 Def Eviolite Clamperl: 15-18 (75 - 90%) -- guaranteed 2HKO
Possible damage amounts: (15, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16, 16, 16, 16, 16, 18)
Regarding the most common scarf set, omanyte admittedly happily sets up on rufflet locked into bb or return or u-turn and takes little, while clamperl takes a lot of damage. However, since rufflet has access to u-turn, opponents can prevent omanyte smashing by nabbing kills on weakened mons with u-turn to prevent set up or clicking u-turn on forced switchins. I think omanyte overall sets up better vs rufflet but the fact that clamperl also can do this is great for it imo.
Taillow:
The most common mixed lo set bops omanyte with hp grass.
196 SpA Life Orb Taillow Hidden Power Grass vs. 76 HP / 0 SpD Eviolite Omanyte: 21-26 (100 - 123.8%) -- guaranteed OHKO
Possible damage amounts: (21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 26)
Clamperl takes any hit so can set up on it.
196 SpA Life Orb Taillow Boomburst vs. 0 HP / 0 SpD Eviolite Clamperl: 13-17 (65 - 85%) -- guaranteed 2HKO
Possible damage amounts: (13, 13, 13, 13, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 17)
0 Atk Life Orb Taillow Brave Bird vs. 0 HP / 76 Def Eviolite Clamperl: 9-12 (45 - 60%) -- 12.1% chance to 2HKO
Possible damage amounts: (9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 12)
The physical facade set isn't common at all. Taillow in general isn't a flying type you often see, but if you do face off against it, omanyte can't set up on it, clamperl can.
Pikipek:
Neither set up on non-scarf due to bullet seed. Regarding scarf, omanyte sets up better on scarf sets, since it takes far less from return and bb. Clamperl takes any hit from pikipek barring bs relatively well but omanyte sets up better against the scarf set. Similar to rufflet, pikipek has u-turn to prevent being set up bait.
236 Atk Pikipek Brave Bird vs. 0 HP / 76 Def Eviolite Clamperl: 9-12 (45 - 60%) -- 68.4% chance to 2HKO
Possible damage amounts: (9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 12)
Wingull:
This is the only flying type where omanyte has a distinct advantage over clamperl. Clamperl takes a lot from lo hurricane.
236 SpA Life Orb Wingull Hurricane vs. 0 HP / 0 SpD Eviolite Clamperl: 13-17 (65 - 85%) -- guaranteed 2HKO
Possible damage amounts: (13, 13, 13, 13, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 17)
Clamperl almost guaranteed dies to SSSS after rocks.
236 SpA Wingull Supersonic Skystrike (185 BP) vs. 0 HP / 0 SpD Eviolite Clamperl: 16-21 (80 - 105%) -- 6.3% chance to OHKO
Possible damage amounts: (16, 18, 18, 18, 18, 18, 18, 18, 19, 19, 19, 19, 19, 19, 19, 21)
Omanyte takes far less from hurricane and SSSS so omanyte sets up far more easily on wingull.
Archen:
Special archen is rare but hp grass bops omanyte.
Regarding the physical offensive set, clamperl sets up against it
180 Atk Archen Acrobatics (110 BP) vs. 0 HP / 76 Def Eviolite Clamperl: 10-13 (50 - 65%) -- guaranteed 2HKO
Possible damage amounts: (10, 10, 10, 10, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13)
Omanyte takes more damage from eq.
180 Atk Archen Earthquake vs. 76 HP / 0 Def Eviolite Omanyte: 12-16 (57.1 - 76.1%) -- guaranteed 2HKO
Possible damage amounts: (12, 12, 12, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 16)
Both omanyte and clamperl take the exact same amount from head smash so there's no distinguishing them there.
180 Atk Archen Head Smash vs. 0 HP / 76 Def Eviolite Clamperl: 15-18 (75 - 90%) -- guaranteed 2HKO
Possible damage amounts: (15, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16, 16, 16, 16, 16, 18)
Overall, I think clamperl has a slight edge over omanyte just cause archen can be the rare hp grass with spa investment.
To summarise that list, even though evio clamperl doesn't resist flying, it sets up more consistently against most flying types and barring a few exceptions, takes less or a similar amount of damage as omanyte in order to set up. The only times omanyte has an advantage is against wingull, scarf piki not locked into bs, scarf rufflet not locked into superpower and scarf vull. However, all four mons above carry u-turn, so they can prevent being set up fodder by nabbing a kill with u-turn or killing a sack which doesn't die to hazards with u-turn
You say Clamperl's Defense stat "isn't great, especially when compared to other SS sweepers." Firstly, in general Clamperl's Defense stat of 85 is great, being the 24th highest Defense stat in LC. And more importantly, its Defense is fine compared to other Shell Smash users. It runs almost exactly the same bulk as Omanyte - the only difference is that Omanyte has 1 additional HP. Shellder is admittedly 2 points more bulky in Defense, but once again, Simbo has already explained why Eviolite Clamperl is not completely outclassed by Shellder:
When compared to shellder, the niches that clamperl provides are not huge but shellder is A for a reason. I highlighted a few advantages in my previous post about clamperl but to reiterate:
Evio clamperl takes special attacks better than shellder due to 3 more spdef points so can set up on stuff like lo staryu tbolt, non-eball lo abra, volt switch chinchou, +2 vull and probs some other stuff. It also doesn't fear scald burn.
Evio clamperl avoids any form of wa 50/50s. The most notable one is staryu and wa onix but kabuto and omanyte can take +2 icicle spear with wa and proceed to revenge shellder. Ice shard on shellder negates these 50/50s but then shellder has issues with mag and mudbray.
Evio clamperl ohkos common bj mons at +2 straight or after rocks (barring ferro) so clamperl does not fear random bj mons like shellder does.
Evio clamperl is not affected by intimidate.
With shell armour and 100% accurate moves, clamperl is far less likely to get haxed. Crit can ruin a shellder sweep and rock blast missing is annoying but clamperl avoids that.
While I don't think Eviolite Clamperl seems quite as good as Simbo is making it out to be, it does appear to have a somewhat notable niche. However, I'm not sure whether this warrants a rise to B- or not - I'm leaning towards no.
Also it seems that C+ needs some cleaning up because Salandit and Pumpkaboo-Small are certainly much better than Bulbasaur and Riolu.