1. No license is needed, open source in this context just means source available. This has been said in other threads.
2. Yeah it needs to be available for the duration of the jam, otherwise I wouldn't be able to see it. What happens after the jam ends I don't care.
3. Your repo should have everything needed to build the project in whatever context you made the game. If your game is made with Unreal, I should be able to build it after opening the scenes in Unreal. Otherwise I'd be asking for you to host the entire Unreal Engine which you have pointed out is ridiculous.