I'm sorry to hear what's going on. I completely understand about the no-vet-bill choice. I worked in dog rescue for many years, and sometimes those choices have to be made when the money that could be spent on a vet simply doesn't exist.
If it were my cat, I'd be doing pretty much what you are. Suppliments. Vitamin B especially.
I'd add some activated charcoal if you can get him to ingest it - just in case, since poisoning is always a possibility. It won't hurt him if he doesn't need it. But at his age I don't think I'd force-feed him either. (If he ingests some charcoal then expect his bowel movements to be black for a while, don't mistake that for a new symptom.)
There is a one easy test for several of the more common neurological issues pets can get. Put the cat in a supported standing position with just a tiny bit of weight on each foot, then take each foot (one at a time) and flip it upside down. (Do each foot front-to-back, not side-to-side.) If on any of the feet the cat just holds still and does not correct the foot position, then the cat has a definate neurological issue going on. It is not a diagnosis of which neurological problem or how to solve it, but it might tell you the problem is in the brain and not another type of illness.
The pupils can be from a lot of things, so I don't think they really help with the diagnosis.
And I think your instinct of separating him from the other animals is on target. A smaller room, warm, with water to drink, blankets to lay on and a nearby litterbox sound right on target for him. Darken the room if you can, to limit any discomfort from the lack of control he has over his pupils.
And hang in there. Another day ought to bring a lot of answers.