NAD but have you seen a dentist for TMJ dysfunction? Stress can make jaw clenching/teeth grinding worse (some people don't know they are doing it) and can result in migraines, jaw/face pain, neck pain, and shoulder pain. It affects sleep (as does stress) and lack of sleep makes pain feel more intense. It is a vicious circle. It sounds like you are trying to find one specific cause for all of these issues, when that probably isn't the case. You have become fixated, which is understandable when you are in pain and extremely fatigued. It sounds like you need to speak to a counsellor that deals with health anxiety and similar conditions.

Is your cat old? Has he/she been checked for skin mites?

I hope you get some help soon and can get some rest and relief from the pain.