Well i did answer your question because i already tried a lot of the stuff that you recommended. I did a gluten free diet, i dont eat Sugar in general and I did a 3 days Water Fast back in the days without any changes. I also changed my toothpaste etc.
Thats why i said i dont think its an allergy cause i guess while i was taking Accutane I peeled of the dead skin and caused a trauma. Now my body doesnt know how to rebuild healthy skin on my lips. And unfortunately thats something that i personally feel is not curable, since no one got cured who just damaged the skin over time.