That is quite a list. Sorry I haven’t read about this horse before and have a tendency to blurt out what I am thinking.
The weight bulder and the hard keeper solutions are the ones I would have suggested to try along with extra vitamins and minerals.
Perhaps there is some key micro/macro mineral missing to help him better absorb the calories that he does take in.
Quite a few years ago my mom’s horse had a stroke at 22 (vet thought it was a stroke) and one side of his face didn’t work very well for nearly a year. Keeping his weight up was really hard.
We used the above mentioned supplements, but I also added an over-all super duper vitamin and mineral formula (forget which one, but ordered it from ValleyVet) along with Source Senior (or just organic kelp) for even more micro-nutrients.
Like you, she was feeding a slurry of senior feeds, beet pulp, oils, cider vinegar and weight gainer supps, but after we added all the extra vitamins and minerals, there seemed to be a better slow and steady weight gain.
She also would mow a section of pasture for him, then re-mow it and re-mow it to bits, until he didn’t have to chew it hardly at all. Then she either fed it right to him, or put it out to dry in the sun, like hay, then stored it kinda like hay. She made sure he had some in a tub/big bucket available to munch on all the time. But he had other health issues and was not on pasture at the time either.
If your guy is on pasture, maybe he is getting just enough to satisfy him mentally, but not able to chew and graze enough physically for his body.
I dunno if that helped… I’ll keep pondering, it has been quite a while since we did this for her horse and I can’t remember very much right now.
Aside from his (mom‘s horse) very specific heath issues, the vet would often say he was the healthiest sick horse he ever saw. I want to say it had something to do with getting every other part of him healthy and adding all the vitamins, minerals, and micro supplements that helped him to better absorb what actual nutrition and calories we were able to get into him.