You really have been through a lot. Like you said on my thread, we are in similar situations as they have listed me as indeterminate colitis as well. I have always had every sign pointing to UC; whole colon involved, no patches of healthy colon, no small bowel involvement (except for recent biopsies), rectal involvement, pain, blood, urgency etc. I also got the Prometheus test and it pointed towards UC, but again it cannot be 100% for certain.
Surgery would seem like the obvious choice in your situation if it weren't for the uncertain diagnosis. I know there are a few other tests that can be done to try to get a confirmation.. I have had a CT enterography and I am going to get some other fancy test done soon where I have to drink some barium contrast and they take pictures of the sm. intestine. I am not sure of the name of that one but there is also a capsule endoscopy, where you swallow a pill that has a camera and takes pictures of your sm intestine as well.
Maybe some of those tests could help and I hope your colonoscopy coming up gives you some answers!
Feel better