Grass Knot is used so that we can avoid getting Pursuit trapped by the Tyranitar that is paired with most Keldeo, CBTar, as after SR we have a solid chance of OHKOing it... Grass Knot vs CBTar: 82.1 - 97.1%, 62.% chance to OHKO after SR
Yes, it's true that Celebi fairs better against Tyranitar, but are we really going to rely on a 62% chance to beat Tyranitar? If it's Choice Band (which is by far the most common Tyranitar set when paired with Keldeo) then it's Pursuit does 39.89% - 46.81% to Celebi if it stays in, add having to switchin to Stealth Rock, 2 turns of Life Orb damage if we get unlucky with the 62% chance, and 2 turns of sandstorm damage, then we're looking at 39.89 to 46.81 + 12.5 + 10 + 10 + 6.25 + 6.25 = 84.89 to 91.81. This means that Celebi will either be dead upon switching back in to Stealth Rock or, in the best case scenario, we're withing KO range of Keldeo. Meaning, we're literally relying on a lower chance than Focus Blast hitting to not lose Celebi and get mauled by Keldeo.
Also, Grass Knot is kinda terrible against pretty much anything other than Tyranitar. It has like 20 BP against Rotom-W, meaning Rotom-W really isn't that scared of our Celebi, yes we have Psychic, but it will not do very much to the standard specially defensive variants, which can then Volt Switch out to something that threatens Celebi. This means that Celebi is not very good as a Rotom-W check (and if Celebi replaces Latias it will be our primary one) because if Celebi comes in on Rotom-W then the opponent will just see it as a free switch and then gain momentum (which is very important in the current meta.)
And Celebi fares just fine against Thund-T as long as it avoids switchint into HP Ice. Psychic does 70.56 - 83.61% to Thund-T, 62.5% chance to OHKO after SR, and sure OHKO after SR and 1 LO round.
Avoiding having to switchin on Hidden Power [Ice] is easier said than done, if the opponent predicts correctly, we'll have to sac something every time Thundurus-T comes out because it does mass amounts to all our pokemon while we can't do much in return with the correct coverage move. Although this is unlikely it's still possible that your opponent will predict perfectly and if Celebi comes in on Hidden Power [Ice], it takes 56.91% - 67.55%. Thundurus-T is going to do a ton of damage to Celebi even if we switch in on something like Thunder, and it's very likely Celebi will be weakened to the point it will barely be able to switch in on Stealth Rocks, let alone an attack. With Celebi's lacluster speed it will have a very hard time finding an oppurtunity to come back in and use Recover, and having to sac our main water resist on a threat that is often found in rain, with our only alternative being saccing something else unless we make really risky switches is far from reliable.
Maybe, just maybe, we could replace Terrakion with Sash Alakazam? This way we can revenge kill everything fast at least once, covering Venusaur too, and still have a good cleaner. And to cover Volcarona we can use T-Wave on Zam. So a set of Psyshock / Focus Blast / Hidden Power Ice / Thunder Wave could work maybe
Sash Alakzam is pretty cool back up / emergency check yo many things, but it shouldn't be used as a primary check to anything. The reason for this is that it simply can't switch in on the likes of Venusaur and other sweepers, if it switches in on a move then it's screwed, we'd have to sac something pretty much every time Venusaur comes out for this to work. If we use Thunder Wave as our only way of checking Volcarona then what happens if it's Lum Berry? Or sub? Or maybe even Chesto Rest? In that case we'd be completely and utterly screwed. Alakazam also shouldn't be used as a primary check to anything in case the opponent has 2 sweepers on the same team, we'd have to choose which one to check with alakazam, making a sweep pretty likely.
I also think Celebi makes us even more rain weak, it takes (38.30% - 45.48%) from +0 Keldeo in the rain, and 57.71% - 68.09% from Specs Keldeo in the rain, an easy 2HKO. Although Latias takes similiar damage, the difference is that Latias is way faster than Celebi, it can switch in on a hit from the likes of Keldeo and then outspeed and OHKO. Celebi's also much worse against Tornadus-T, Thundurus-T etc. And although it's true that Celebi fairs better aginst Ferrothorn that Latias, the calcs you listed are barely 3HKOs under rain and if Ferrothorn carries Protect, then Hidden Power [Fire] is only going to be 4HKOing, resulting in Ferrothorn still getting up 3 layers of hazards. Overall, Celebi may be better than Latias against Keldtar, assuming we're lucky enough, and it's a bit better against Ferrothorn in the rain (even though I'd still say Taunt Gliscor is a better switch-in) but it's weaker to everything else. I'm not trying to say Celebi is bad, but I think Latias is better for the team.