Sounds like you got everything covered.
I'd ask the vet if she/he thinks it might be IB. Sure sounds like it might be a possibilty. Has your vet considered putting them on venison (spell?) aka deer product. It's a vet rx food and is used allot with cronic potty problems. Its not cheap, our vet suggested it for Skinny and I tried other foods first (Nutro) which worked for us. I was glad we didnt have to do that venison route.