Yes

That should be fine, though a PWM controller might be better.

Theoretically yes? But I’m not sure if there is something funky about the way motors work.

Only one way to know for sure…

Almost… You need a 12v transformer, power supply, wall wart, or whatever you want to call it, rated for at least the total current drawn by all the fans you plan to power. (Really should shoot for at least double). So if your are running 5 fans, aim for 3A or greater on the p/s.

Edit: Meant to reply to top level