Instead of attaching the motor bracket with a binder clip, consider sticking it down with some sort of removable adhesive?
3M command strips should be very strong/secure, while also allowing for adjustment and will detach without residue when the time comes.
I'd cut a block of wood to fit snuggly to the left of the knob, wedge it in place and screw the stepper in to that.
.....or just screw into the AC unit somewhere discrete and hope no one notices.
Just putty over it and get a slightly matching high-gloss or semi-gloss enamel and nobody will notice it. I've put tons of holes in things in rentals over the years and basically nobody will care as long as you restore it to like condition when you leave.