Bike

Vex X3M. It teaches the lean and flip timing on forgiving courses, and everything else in this tag assumes you already have that feel for the bike.

Rotation, not speed. The bike needs to be roughly level when the wheels touch down, so a flip that looked complete in the air usually needs a small correction held on the way down.

Not at all. Each is self-contained and the handling barely changes between them, so you can start anywhere - the later ones simply expect a bit more precision from you.

Grip. Moto X3M 4 Winter keeps the same bike and controls but puts ice under the wheels, so braking distances and take-off timing both need adjusting.

Yes, all of them run free in your browser with nothing to install and no account needed before you start, and restarting a course costs you nothing but seconds.