1. Under the rules section: "Teaming up is allowed, and encouraged! Our recommendation is that you stick with 4 people maximum, otherwise you'll have a hard time coordinating in just a week.", so it's not mandatory to have a team but it's overall better
2. Also under rules: "Apart from that you are allowed to use any game engine or framework, you can also use assets not made by you, but make sure you have the proper license to use them and crediting is of course encouraged!"
3. Yep, when the jam starts