Will you reveal a theme for the jam? or is it gonna be just the limitation of 2 buttons? if no, is it "legal" to start the game early?

Can I make a game with one button and mouse movement?

There is be a theme unveiled at the start, yes !

Three words, you can use whatever of them. That will cover several concepts, so I guess you can start some technical work early, it's up to you =)

But one button and mouse movement would count as 3 buttons (2 axis for the mouse).