You’ve received some wonderful feedback here. I’ve fortunately never dealt with your type of situation before and I’m so sorry you are going through it. It sounds like you’ve given it your very best effort to figure out what’s going on and help your young horse.
I did have to euthanize my 7 yo OTTB exactly one year ago this month. She coliced and declined very rapidly before we could even get her on the trailer and to the university. My vet said euthanasia was the kindest thing I could do for her at that point. It was a horrible decision to make but the right one. I was present during the procedure and glad to be there (I understand some can’t see that).
Make sure your horse is in an easily accessible place (easily accessible by machinery). I didn’t even think about this when my vet recommended we move her to the round pen and out of a stall. We would have had to tear down part of the barn to get her out otherwise. My vet also sedated her, let her fall, and then proceeded with the euthanasia. It was heart wrenching. There’s no way around it.
Good luck with whatever you decide. I think it is extremely commendable that you’re not willing to sell her off or even give her away as the outcome is never good for a horse like you describe. Hugs and best wishes to you.