How is the post game?
Dunno, but I've heard it's in-line with the first games of the other gens (see: pretty light.)
Is Mega Evolution still a thing? Can I still use my mega stones?
Yes, but not in official tournies. Z-Moves and Z-Crystals are the new hotness.
How soon into the game could I transfer stuff from the Bank?
Sun and Moon haven't been enabled for Bank yet (will sometime next month I think.)
Now that the game is mostly 3D, can I actually see my pokemon when I'm surfing? Or does the player still ride an indistinguishable blob?
HM's are no longer a thing. Instead you get 6(?) "ride Pokemon" that are specific: Charizard for flying, Lapras for surfing, Stoutland for hidden item sniffing (AKA dousing machine), Tauros instead of a bike, etc. You see these Pokemon in the overworld while you're riding on them.

They don't count toward the Pokedex, don't take up team space, and you don't actually catch them - they're given to you like an item is.