Oh yeah no, that should've ******* put Josuke on his ass, hospitalized, if not straight up kill him. That ain't healing, but it's still a good feat for Josuke as both a stamina thing and how much damage he can take before getting put down, given that's a normally fatal wound, that was like 3 inches of shrapnel in the back of his head.
As for that, hard to say, reforming is just how the Stand works, (Kato for example was reduced to literally a puddle) and they all took some hefty wounds but were fine after the fact.
But surviving the liver thing is notable, I would chalk it up to it just being apart of the reforming thing, but that's clearly not how it works given Kira died from that even after they escaped Damo's effective range in the flash back from the very same arc (Then again, Norisuke had his arm cut off...).
So yeah, that might be healing, or rather, a pseudo work around via fusionism that might be legit, as long as we explain the mechanics of it and the ambiguity, kinda like Sticky Finger's "healing" via zippers.