If you submit the game and then make update, you can simply update your submission. From what I know you can even update it during the voting phase.
Of course even after the jam is over you can continue t create updates to your game.
As for preferred format.. I would say windows or in-browser. Most of the people here use windows (I think) so it will work with most of the "voters" and if you want as many people to check your game then create built for browser. It's easier to check it if you can play in browser instead of downloading it.
Believe me, there will be a lot (and I mean A LOT) of games to play.