Obviously Gaban can counter regen thats why Chopper went to ask him and we actually saw on panel Sommers couldnt regen after gaban attacked him
But how did sommers get up? Did imu heal him? Or you can simply put them together similar to how Law's fruit work?
I think we saw the mechanism quite well.
Imu can repair individuals that are under their influence (demon or 'basic' HK - more to that at the end)
Broggy's arm was separated but after getting influenced, the problem was fixed. His main body's functionality given. Even though a part of it misses still.
Sommers was quite out of order. And imu, being in close proximity brought him back to the original state. Without the pentagram, since they're already connected and the Imu power can channel directly to him.
That said, I suppose once individuals are made evil, they're technically dead and it's their souls or whatever that's controlling the body. The soul that has been corrupted. Dorry and Broggy won't be coming back. That will be a key in defeating them - they need to be eradicated fully. Maybe by an ikkoku homage in respect to them. Gaban might give this crucial info once he returns later.
Regarding the HKs, I suppose they have a full demon form as well. You probably can learn to suppress the demon stuff and turn it down so you look like a normal person (no horns, bulging veins, wings, normal size). It's certainly convenient and would dramatize the final battles on Elbaf (or anywhere else) even further.