1. Keep things simple one main mechanic is a good idea as less complex games are easier to pick up and play and it will leave more time for you to polish out the game.
2. Few minutes of gameplay is more then enough we have a lot of games to review and we just need to see what the game is about.
3. Don't upload at the last moment not only can you drop a link for people to play test it and fix anything before the jam start and upload new fixed build you also will have time to work on the cover image and screenshots if they look good you'll have a higher chance of beaing rated and having less then 20 ratings lowers your score.