itch.io

It can be standalone - but multiple players will visit your at scene at the same time.

Only one game would be chosen for one of the four main prizes. But your other games would be in the running for the top 25 prize of 100 MANA

But by all means, if you'd like to make 3d arcade machines that open a 2D UI, great!

Decentraland is a 3D environment, so you'd need to consider how 2D would work in this space. Take a look at some of these example open source scenes for inspiration https://studios.decentraland.org/resources

Yes, absolutely!

No, when the Game Jam submissions go live, you'll get the option to add your team details when you submit :)