Every step sounds pretty good in winter - except using a sledgehammer to drive 6x6s into a mucky/gravel bottom.
I do not know of any builder that would construct a house deck using support posts set in loose fill. I think those would be set in an oversize hole that is backfilled with cement, or attached to anchors on poured concrete post bases. I would definitely apply the same construction rules to a pond dock, minus the concerns about frost-heaving of the support posts.
Further, if you are only working above the ice surface, then your dock and posts will be subject to "racking" failure if you do not have any crossbeams set down low on your posts. If your bolted 2x4s are going to be sufficient to stop the racking, then I think you will have a very difficult time driving your posts. The connected post and crossbeam are going to resist downward movement of the post you are attempting to drive.
I am pretty sure that you are a much better builder than I am, but those would be my concerns about your dock design.
Good luck on your project!
