Hi @lindaray. You’ve certainly been through the wringer with little ‘ol tooth #3 with a new crown, root canal, an apioectomy and the possible side-related issues in your mouth. You gave very detailed and helpful accounting of what’s transpired over the past year with that tooth.

With the deterioration of the tooth under that old crown from 20 years ago and the pain you were having in the tooth, it’s a possibility that it was already in the beginning stages of dying. It can be difficult to diagnose at that stage and often doesn’t show up on an xray at the apex of the tooth until there is an actual infection. So your dentist, at the time, may not have been aware.

When the crown was placed and it felt off, it may have been due to the tooth (nerves in the tooth) being dead. The pain associated with it could have been from the abscess forming on the apex of the tooth and the tooth might have felt ‘off’ because of lack of sensation with the dead nerve tissue.
It sounds like the infection with the root canal was a little challenging with its recurrence in one of the roots, requiring the apioectomy. Were you given any antibiotics at the time of the root canal treatment or when the abscess was first discovered?

Fast forward to now, with the symptoms of dry and burning mouth that you’re dealing with. That’s a miserable condtion!
I’m not a dentist but as @gingerw mentioned, my career was in the dental field for 30+ years. So I’m digging into experiences I recollect to relay some possibilities. While not common, it is possible for patients to have allergic responses to the material used in the permanent crown. Also not common but possible is long term inflammation from the abscess and root canal treatment which is initiating an autoimmune response. I can recall 2 occasions when patients had long term issues where the resolution was the removal of the offending tooth. Which I’m sure you’re reluctant to do because of the money invested in it already. Again, I’m just tossing these scenarios to you as examples of actual patient experiences.

First it’s important to find the underlying cause of the inflammation. My suggestion is to try your primary care physican, a rheumatologist or a Functional Medicine practitioner to check to see if you have inflammation in your body. This is done through simple blood work and then work from there. The doctor may be able to help you discern if there is any correlation between the tooth and your inflammation or if this triggered an auto immune disease such as S’jogrens symdrome. Which a rhuematologist will be able to diagnosie.

If you’re one to do research this article discusses the potential for autoimmune responses with an abscessed tooth. From PubMed:
Root Canal Infection and Its Impact on the Oral Cavity Microenvironment in the Context of Immune System Disorders in Selected Diseases: A Narrative Review https://pmc.ncbi.nlm.nih.gov/articles/PMC10298853/

I have burning mouth syndrome related to a medical condition and when I have flares I avoid toothpaste with mint and use children’s toothpaste. I also switched to a super soft child’s brush. I avoid ice, condiments with acid, salt, or hot spices. One thing that seems to offer a level of comfort is ‘Pulling” with coconut oil. 1 teaspoon or more of organic coconut oil. I let it melt over my tongue then swish vigorously around my mouth for 20 minutes. Then spit into a tissue. Not the drain because it will clog eventually.
I hope you find resolution soon and please let me updated, ok?