Paint guys dont want to deal with brake fluid or bleeding. Mustang guy probably wouldn't want to pay for it either, or he would have just had them powdercoated
really no need to, unless you want to be meticulous. But if that's the case, then you would actually be powder coating. This is supposed to be an easier/cheaper alternative to powder coating. But this way you don't have to dismantle each caliper, thus saving you a lot of time and energy.
Because it helps the paint stay without chipping if primer not used. Also paint can diminish if there isn’t a complete flat surface so the primer helps create that flat and clean surface