The basic idea is once voting begins, you don't want to change it up so people at the start of the voting process are playing/voting on the same game as people at the end. Bug fixes are 100% allowed (and encouraged) but no features should be added once voting begins. BEFORE voting begins, you can update it however you please as far as I'm aware. "Submitting" really just links to the game's page, not the exact version you submitted.
As for the format, web-browser is preferred if feasible since people won't need to download anything and can just play it right away. If not, then Windows is likely the best bet. Linux folks will just run it via Wine or something similar, and no one cares about Mac users (kidding! ;p ). Godot thankfully makes exporting to multiple formats easy so nothing says you can't export Windows, Linux, and Mac and have all 3 versions on your game's page.