Pretty sure this a straightforward question but some people at my venue disagreed, so here I am...

If Silver Banshee uses her SP to Incapacitate somebody, and then deals damage to them based the # of action tokens they have (as per the SP), would she receive Mystics feedback damage if said target was a Mystic themself?

Oh. and this just occurred to me (totally separate from the above situation). If you use Stunning Blow to hit a Mystics guy, do you receive feedback damage?

Yes she would as they are being Dealt Damage from her attack.

And yes, Stunning Blow would also deal Mystics Feedback Damage.

No.
The action, and thus the attack, is resolved before the damage is dealt. The damage is simply from her power.
Yes, as that damage comes from the attack.