Hi,
FWIW, I just logged on with a similar question. My right jaw, where I had my radiation about four months ago, is getting stiff and sore when I close my mouth, and my teeth don't want to come together the way they used to, especially in the mornings (I wear a night guard that holds my jaw slightly open all night long). I've been told this is almost certainly from the formation of scar tissue in the area of the jaw.

As for opening my jaw, I can get three fingers, which I think is pretty much where I started (maybe a slight reduction). I only had radiation and chemo, no surgery (so far).

47 yr old male non-smoker, social drinker, fit. Jan'10, Stg3 rt tonsil+rt neck SCC, HPV+, rad+chmo Vancouver Cda. 2yr clear Apr'12 London UK. Apr'13 mets recur to lymph btw left lung & aorta, 3x Cisplatin+5FUchemo+20 rad, was all clear but 6-mo PET-CT shows mets to pleura around left lung, participating in St 1 trial of GDC-0980. GDC lost effect and ended July'14, bad atrial fibrillation requiring hospitalisation, start more standard chemo 10 Sep 2014.
Sadly has passed away, notified Jan 2015.