Brainer:
If he can just slide right up to a dude, no problem, right?
If people are shooting at him, or dude is running around and the Brainer has to chase him down, that sounds like acting under fire to get it done, to me.
If the dude punches him in the face when he goes to tag him with the injector, he might have to seize by force, or just act under fire, depending on the fiction, I think.
Angels:
Assuming the Angel can use his kit on himself in the fiction, it's all good. If he can do any of the things it says he can do, like narcostab himself and zone out for a week, if he still has hands to use the kit after getting injured, of course he can treat himself. If other PCs know what they are doing, of course they can help him.
Healing Touch:
Yeah, on a hit, healing happens. If you take that away, somebody's not getting the full benefit of their moves. If the Angel fails to act under fire, though, all sorts of stuff could happen:
-- Take away their stuff: Now he can't use Healing Touch anymore! What happened? Maybe he must extract his energy from the patient and put it back in his own body.
-- Announce future badness: He could have visions about all the bad stuff the patient is carrying around. Maybe the patient has done a lot of people wrong, and the Angel can't help but see that it's going to come back in a big bad way.
-- Put them in a spot: Give the player a choice: "You're about to pass out from the strain but if you hold on, you're going to fuck up the healing and your patient wil be permanently marked because of it. Is that cool or do you finish your work, no matter what?" Then let the player choose. Does he really want to be unconscious for the rest of the firefight? What does it mean for a patient to be "marked"?
Infirmary:
Sounds cool to me, that's one reason to have assistants! If the hardholder gets to roll+hard to have her gang go and seize a fortress by force when she's not there, it seems to me the Angel could heal people by getting his assistants to do it, without being there -- or as the patient.
Reload:
Key words here: "It looks awesome on screen, of course, and it makes sense." If it makes sense, that's the important part. Do it the way that makes sense and is awesome. There's no "rounds" or anything, and a "turn" is just somebody's turn to say stuff. If you ask a player what they do and they say "I drop the empty machinegun, whip out my pistol and shoot Wisher right in the face," cool beans! Roll dice for it and let the dice tell you if Wisher is quick or dead.
Does that make sense?