1: For the supply quest you get an extra 50% (I think) rewards for turning in an HQ item - the 200% bonus is just for levequests.
2: Any of the Grand Company Levemetes out in the field will accept supply and provision turn-ins. However, I am not sure if it is only members of your Grand Company or if any will do. Also, all of the supply/provision quests are shared no matter where you turn them in so you can't turn in multiple items for say Blacksmith to different NPCs.