I have been reading through this thread. I too have had dry mouth and dry eye problems, reflux, dry cough, thick phlegm, breathing issues. Over the years I have corrected many of the issues like reflux (gave up gluten). Dry mouth and eyes increase my fluid levels especially before I go to bed (drink 12 oz of water). Taking allergy meds for cough, etc.
I still had dry mouth and cough with very thick choking phlegm. I had major dental work on my back molars when I was 12 years old ( I am now 70 years old). 10 years ago I had the major deep filings replaced with a composite type material. Still had the sinus coughing phlegm issues and they seemed to increase. I have been reading about biological dentists and had found a dentist in my area. I have a first appointment schedule for mid July to have a cone beam CT scan of my head and sinus as well as teeth. I am pretty sure I have some sort of infection that is causing my problems.
In the mean time last week my back molar broker off and exposed the original tooth at the gum line. The tooth is not bad doesn't hurt so I am waiting until my appointment with the bio dentist to get it looked at and repaired. That was 7 days ago that the composite material came off. The last few days I have felt better than I have in years, The coughing has stopped. The thick phlegm has disappeared totally. The funny taste in my mouth is gone too.
This leads me to believe I am on the right track in getting the cone beam CT scan. I suspect the tooth was leaking into my sinus area on my right side. The daily headache in my right eye area has disappeared too.
Long story but I would suggest getting a cone beam Ct scan and have your sinus areas and teeth check for inflammation. It should expose any problems around your crown area.
This is just my comments and thoughts after reading your post. I am always searching for solutions too,