I’m very sorry you lost your bun, and in such an upsetting way. It sounds like maybe his heart gave out. There probably wouldn’t have been anything a vet couldve done. He didn’t die alone, you were there for him.
He seem’s to have been ill a long time. I’m sorry you couldn’t get a proper diagnosis for him. This is not rarely the case with rabbits. Really good rabbit vets are few and far apart.
It could’ve been ec as you suggest, because it’s a disease that can affect many organs. It can affect the nervous system, the kidneys and the GI system. It is of course also possible that he could’ve had a brain tumour, because all animals can, it’s just very rarely diagnosed in rabbits, so we don’t know how common or uncommon it actually is.
If your bun was hungry and eager for food, I’d say there was something going on that consumed more energy than he could eat. Pellets are energy dense so even if he did eat less hay, that wouldnt explain the marked weightloss.
I’m glad he did enjoy his daily food though, it means he wasn’t in a lot of pain even though he was sick. Pain causes inappetence in rabbits, and rabbits are very pain sensitive.
Again, I’m sorry you lost him. It seems you really did try to find out what was wrong with him. We can’t do more than that.
Binky free, little bun.