this is my first jam too and as far as I can tell, multiple submissions are probably ok because I know of at least one participant with 2 entries. As for the update once it's submitted, I imagine that's a perfectly natural part of the process. That's why a game jam has a limited timeframe, to build projects before the ratings and eventual announced winner kicks in.
I, personally, have done like 7 updates since I first submitted my project...not that the submission feed would reflect that in my case. Unfortunately the feed seems to exclusively take issue with my project which is why you don't see it in the feed. Point is, updating your project before the 22nd should be not only fine but encouraged.
