Permission from saman to comment here, sorry for taking long, had work.
in my point of view, the third level of the low to high categories is always a sorta of bridge to the next one, low deals with small wounds but high-low has minor organs damage, mid deals with important damage but high-mid allows you to make a body from a tiny piece and high deals with rebuilding your body from physical elements but the higher level of high uses energy instead.
So in this case, the difference between mid and high-mid is that mid is still dealing with recreating parts while high-mid is starting to deal with rebuilding, both use heads but one recreates just the head while the other creates the body from the head.
IMO i think recreating your body from your head should be part of mid instead, but a higher form of it, after all, mid can already deal with everything from below your neck, so being reduced to a head and healing it is just a somewhat wider form of it, not something more complex, while high-mid is for the same but when the piece is even smaller and less significant.
I think making it so the brain has to be destroyed alongside the body would fit high-mid, since now you need to deal with both the loss of the brain and the organs at the same time while relying on something irrelevant to your fuctions like a toe.
Also i think minor brain damage should be on low-mid, is minor for a reason, if it doesn't affect the brain fuctions drastically like a coma or your body not fuctioning, or at least a large amount of damage is done, it should fit with the "organ healing" introduction of low-mid.
As for the low category debate, i think wounds that would normally incapacitate and kill quickly you should be low-mid, like that one vein on your neck that if cut just instakills you, while wounds that take time to kill you would be high-low, though this one seems very debatatable.
As for the "fiction inconsistency" stuff, i think weakeness about their regeneration should suffice, though that depends on if i'm allowed more comments after this one, because idk how this works exactly