So say I mega evolve my, I dunno, metagross. Then suddenly a charizard crits me and metagross faints. So my question is, when I use a revive on it, will it still be in its mega form?
Your Pokémon will not be Mega-Evolved as when the Battle ends or your Pokémon faints, you revert to your non-Mega Evolved form. You can't Mega-Evolve it again also.
Source: Just tested it and https://m.bulbapedia.bulbagarden.net/wiki/Mega_Evolution