Yes, the zero replies so far on that forum are building my confidence and area-of-modification-awareness like nobody's business.

Not sure I understand what is wrong with the mouth though. The muzzle is too long? Or is the corner of the lip not far back enough?
But yes, heads is where I need most work; with the head in anything but a classic pose like front or side it usually takes me more time to draw the muzzle than it does to draw the rest of the body... I just cannot get the perspective on it right without a load of trial and error.