Hi Mike,
Sinus problems are closely related to TMJ and usually doctors won't find anything wrong if checked. I wish I could have all the money back that I paid to doctors over the years just to have them say - "nothing is wrong with your sinuses" or ears, etc. It's frustrating.
Breathing with your mouth open all the time is usually not a good idea. I don't know if I can explain why well enough but here goes. One - you need the natural air filters of your nose to ward off germs, etc. Two - keeping the jaw in a relatively closed position does stabilize the joint. That's why we are always told to keep out teeth apart at all time BUT the lips together (or slightly apart) with tongue resting on the roof of the mouth. And a possible third - breathing thru my mouth puts a little more pressure on my jaw joints. They might not for other tho.
Personally I can't see where breathing thru the mouth might change the "shape" of your jaw. It is quite common for the human body to differ in size and shape from the left side to the right side, including the jaw (only a very, very little bit). Is your mouth deviating or shifting when you open and close it?
I would imagine that you have been under a great deal of stress lately.
Stress is very bad for TMJ. Use just about any method you can think of to relax your body. Massage Therapy is a great way in my opinion. Getting good sleep is essential. If you are having problems breathing thru your nose, you might ask your doc for a nasal spray for nights.
Are you wearing a bite splint right now? A good one can really help when we are stressing and clenching/grinding our teeth. A neuromuscular dentist seems to be who you should see. PM DyanMatteson your zip code and she can locate some names for you.
The back-of-the-head "click" sounds like you might be cracking or popping your neck, but I also get a quiet click sound deep inside my head (aside from my neck cracking) and don't really know what it is.
I also just recently posted a list of self-help methods to TenaciousE "Tmj ~ Living in Pain ~ . . . See if any of these might help. Please ask any questions you have. I don't know if there is a stupid question when it comes to TMJ.
Take care and God bless...
Carol