I have a creve too that I suspect is part Polish and he went through a bad bout of wet neck a couple months ago. He would turn his neck so far around he ended up flapping around on his back. To make sure it wasn’t a head/neck injury I brought him into the vet who said there was no indication it was caused by an injury and said it COULD be caused by an ear infection and we had him on antibiotics. I also had him 2 doses of 400 iu of vitamin E per day. I would pop upon the capsule and squeeze it out on a piece of bread and he gobbled it down. I also clipped his crest feathers around his eyes so he could see better (his left eye was completely obscured by feathers.
The vet said he had less than a 50% chance of survival, but like yours, he was eating and drinking and seemed to be very much alive. After 2-3 weeks he snapped out of it and now goes about his business like nothing ever happened.
Another thing I did was when he would have an “episode” was I would let him go through it. He would do a barrel roll on the ground and once he had made the full 360, he would right himself and go on with his day. During the worst of it, he would do this many many times a day. Eventually it got to where he only did it a few times a day then it stopped altogether.
Now for the weird part: except for the few days that were the worst, he ONLY did this a night when the sun started to set and it would get dark and when he would go into the coop or the cat carrier he was sleeping in for a while. During the day when he was out and about with the other chickens, you would never know he was anything but a perfectly happy, healthy chicken.
Stay the course and you never know, he could go on to live a perfectly normal life.