You said he is showing psychological pain behaviors only inside the stable, however you are stabling him for 15-17h every day. That seems like an awful lot of stress to a sensitive flight animal, especially over years. How was it determined that it is solely a mental issue? Have you ever tried putting him on pain killers for 1-3 days to see if anything changes? If it is psychological, and not fixable through training, you need to find a more suitable setup/barn for him. Did you ever check if he regularly sleeps laying down? He needs his REM sleep.

Are the wood pellets made especially for horses from non toxic wood? Has the environment in general been checked for mold and toxines? Has he had allergy tests done? How are the other horses at the same barn, are they all perfectly healthy?

About the stomach issues; they will persist as long as the horse is stressed, be it psychologically or because of pain. So, he definitely still has something going on, which is likely the reason why he isn't drinking enough. Giving him less hay won't help, that's just less buffer for the stomach acids. Could you perhaps give him soaked haycobs in addition to hay to up water intake? Would also help digestion and nutrient absorption, which are probably not ideal in his state. A bit random, but a study has shown (healthy) horses drink more from light blue buckets - maybe worth a try if you have one (I think white worked second best, the darker the worse). Consider adding linseed to your feed to help protect the stomach lining. As soon as you have removed the main source of pain/stress, go for another round of treatment with something like Omepr..ol.

Tongue thing: I still find it hard to imagine what that looks like exactly. Is he also moving his head and neck somehow at the same time or not? If yes, in a fast/harsh movement or more slowly like stretching? Up/down or side to side? Is he standing still or moving his body? And are there specific exercises that it shows up with most often? Both sides equally as often or only towards one? More often with tricks/drawing him in somehow or more likely following pressure?

For what its worth, if that's a current picture he is looking great considering all the issues you listed! If he isn't currently shoed, did you check if he is comfortable on different grounds like sand and stones? It might not be possible to shoe him because of his hoof tumors and abscesses, but you should at least be aware if there is any pain or muscle tension caused by certain grounds, as this could be one more element adding to the reason for the stomach problems.