I do not have any real help to offer..... but mostly I just wanted to offer my sympathy for what THAT is worth I can share your frustration!!!
hmmmm....it does sound like a visit to both the vet and the groomer *may* be necessary and both will require taking a messy and scared kitty in the car....perhaps that first visit will need to be to the vet where they can check the messy backside - *and* give shots if you chose to go that route. They are going to have to do a bit of cleaning in order to see what is wrong!! Once you leave, you will KNOW...and can then go to the groomers to get the needed clipping if that is all that is needed. If the vet does discover something medical, then I would guess they would proceed to take care of the whole job wouldn't they?? Soooo...sounds like the vet really has to be the first place to go. Could you maybe try calling several vets and see if another might be a bit more....helpful??
You are being so kind in wanting to help this little cat and I am so sorry about the frustrastion and aggravation!

Hope you get some good advice here - beyond my ideas!!