Snivy can stay where it is. Even with Contrary Leaf Storm, it's really easy to wall since all the good Hidden Powers mean it can't run 17 Speed. Glare is great though.
Honestly, the things that actually beat Snivy outspeed it anyway. If you believe Snivy actually needs 17 Spe, then I'm afraid tot ell you, but, you are sadly mistaken. I mean, of course it would love to have 17 Spe while using HP Rock, to speed tie with Archen switch-ins if you used Sub > Glare, but I mean, you don't actually need it. With Glare and Substitute, Snivy has the ability to play incredible mind games against the opponent. Also, just putting this out there, but you can run 17 Spe Snivy with a decent Hidden Power type in Ground, the only thing about Ground I dislike is that you miss Flying-types, like Fletchling, and that's never fun, in my opinion. Snivy definitely should be moving up, and if not to B+, then definitely mid B. Snivy is an amazing pokemon that provides insane offensive support, while still being a very great mon that can do damage to teams, even if Ferroseed is being used. (Side note, but my Snivy actually swept a team vs. a Ferroseed that laid down SR and all Spikes while I got up to +6, and then Abra came in to revenge, but Glare Hax was real) Snivy is fantastic.
But yah, I definitely agree with Chen and Carv staying where they are. Carv is good, but I think A- better suits Carv. In some cases Carv's psoitives outweigh its negatives, but inother cases the opposite is true. Chen is fantastic in being a soft check to several pokemon in the meta, including fighters, birds, Stunky, normals, grasses, and others that I am too lazy to list. But yah, Snivy up let's go.