Mamoswine Exclusive Moveset

His best moveset is Powder Snow + Avalanche. Mud Slap + Bulldoze (edit: not Earthquake) is also good. Ancient Power is only useful for coverage on PvP.

No.
From what I've gathered, Ancient Power is not going to be great with Mamoswine as it will be busy in its role as an ice-type behemoth, and Stone Edge is currently better already. Seems like the general way to go is to evolve one or two during the event for two reasons: first, simply for a collector's item, and second for in case Niantic implements stat buffs for attacks in the future.

I hope not. AP is a mediocre move to begin with, even worse without STAB. It only enhances type coverage vs Ice and bug, which aren't big threats to Mamoswine to begin with. (Fire is covered by ground; flying is covered by ice.)

Possibly the worst of any of its moves.

Last time everyone was complaining about the move set for a special event, it was gengar, and lick was the real prize.

Stat buff for AP has been confirmed.