About customizing the jaw; are you going to slice down the middle of the upper jaw and remove a 'slice', then reattach and putty the pieces?
That has worked for me in the past with a couple of customizations. As a side bonus, that method can be extended to make the entire head narrower (make sure you remove and then reattach the nice crest you made first, though.
Naturally the head would have to be removed from the body first and then reattached, but again, that is the easy part ...
I guess it's not as apparent but I had already sectioned the head just the way you said. But, maybe not enough? Well, I don't think I'm going to do more now. The changes I made now are posted below the original photos on the first page. Modified lower jaw and new teeth. Hope this is better now.