Happy to have you on board IsaLand! It's always great to see someone choosing this jam to make their first published game.
1. Correct! We're looking for 20 seconds of gameplay. If you choose to put something before or after the gameplay (like a menu, a cutscene, etc., it does not count towards the 20 seconds of gameplay. If you put a cutscene in the middle of the gameplay, it does. While these things are allowed, if you can get players straight into the gameplay, that would be even better.
2. If you extend the gameplay with pickups, and the total gameplay time goes over 20 seconds, that would be against the spirit of the jam because it's not a 20 second game anymore. If your main gameplay is 10 seconds and the pickups never cause the total gameplay to go beyond 20 seconds, it's absolutely fine.