1) Can Holton Salm use his ability endlessly, until either focus/hit/crit comes up?

2) Can Vader use his ability to take two evades and then evade twice in one round?

Both covered in the rules. Dice cannot be rerolled more than once under any circumstances. So no, Horton cannot keep rerolling.

A ship may not perform a single action more than once in a turn. So, regardless of how many actions you have you may only perform each one time per turn. One evade, one focus, one Target Lock, etc.