Alas my job went on all night, so only had time for a quick preliminary look at this in which I tried the plan above, applied a single level of SD and then changed the edge flow around the mouth. Not sure it's the holotype solution here, but hopefully it's a nudge in the right direction..
I may have an improved version of that later... but actually the more pronounced corner to the mouth is more what snake mouths do than what you were aiming for.
But we could also try and recue what you have and get what you want, but we really do need to add that extra loop to do it (and another one through the eyes nose ideally). But that is not something to fear, and once we have spread out the ones either side it's already looking a bit more even front to back if not top to bottom... and we don't get much pinching this way...
CBR