Questions about momentum and force as well as normal force/friction

[A.T.]

if we are using a linear shooter (2 counter-rotating wheels and the projectile in the middle) then we add another set of wheels (fully identical to the first set with same RPM and everything). it really shouldn't matter at all to the final velocity of the projectile,

A second wheel pair at the same speed can help, if the first wheel pair fails to achieve the desired speed, for example because of insufficient traction.

similarly it means you don't really need both wheels to "push" since it can't exceed maximum surface velocity anyways.

You do need both wheels to push equally, if you want the ball's center of mass to match the wheels' circumferential speed. A single wheel can only accelerate one side of the ball to its own circumferential speed, while the ball center of mass moves slower as the ball rotates.