I apologize in advance for the negative-nancy attitude here:
Magic Guard Celebi: This is one of the better theorymons of the bunch, but this might not be as much of a significant buff as one would think. As if being a Grass-type was hard enough, it also has Dark- and Ghost-weaknesses due to the Psychic-typing, which is the main reason for Celebi's fall from OU. Magic Guard is a lot like Natural Cure, but overall better, so this isn't a bad buff by any means, it's just that it doesn't help with its biggest flaw in being Dark and Ghost weak. Toxic immunity is nice, but many top tier threats such as both Mega Charizards, Aegislash, Talonflame, Mega Pinsir, Mega Scizor, Mega Tyranitar, Mega Venusaur, Greninja, Gengar, Mandibuzz, and many more are still going to beat it to a pulp.
Volt Absorb Jellicent: This sounds like a good idea at first, but it's not that amazing. With Volt Absorb, Jellicent can beat Rotom-W one-on-one, which is a huge feat on its own, but this will tempt Rotom-W to use Dark Pulse. The extra Ghost and Dark weaknesses also really hold him back from being good. I don't have much to say here.
Grass / Poison Tangrowth: The only 3 justifications to using him over Mega Venusaur is Regenerator, having a free item slot, and the fact that he doesn't take up a Mega slot. He takes physical hits just about as well, and has the same attack, but his special defense is horrendous to say the least, especially for a wall, and his speed is obviously going to hold him back. Having Regenerator is great though, since he's going to switch out constantly from fear of almost everything Celebi's scared of. Having an Assault Vest is also a good idea, since Regenerator is his method of regaining health, and trust me, he's going to be using his ability constantly due to being forced out frequently enough. This will likely get my vote, but I'm still feeling mixed about this one.
Grass / Dragon Sceptile: If Sceptile were a physical attacker and had Tough Claws, or at least had Nasty Plot, then I'd be pretty happy, but this sadly isn't the case. Instead, we give Sceptile a typing that removes one weakness for two more, and magnifies his Ice weakness; not a good trade-off. But offensively, it's almost as bad. Grass/Dragon coverage is stopped dead in its tracks by Steel-types, so his only option to hit them with on the special side is Focus Blast or HP Fire, and even then, Steel-types like Aegislash, Mega Mawile, and Skarmory are still gonna take Sceptile's hits and go about their business like they do everyday. Also, base 105 special attack only goes so far without a boosting move, so Chansey, Deoxys-D, Clefable, Mega Charizard Y, and Mega Venusaur can come in, most can avoid a 2HKO, heal off any damage, and the latter three can hit with super-effective STAB attacks, while the first two can Toxic stall. I really do wish this idea were good, but I just don't see this working.