I am sorry you might be faced with this hard decision. But nothing is definite yet, so don't panic. When the time comes ask your vet about the risks involved with the surgery and the testing that goes along with the medication.
I know nothing about HT, but I did have a diabetic. He was an angel and I could pill him and inject his insulin and he was an ideal candidate...........EXCEPT for when he needed to have blood drawn at the vet for the blood sugar curves. a procedure that had him stay at the vet for the whole day and draw blood every few hours to see how his levels were doing. He was a holey terror. Many a time I was asked to come get him out of the cage because he was upset. In the end, this is what helped me with the decision to PTS rather than do FURTHER tests when he became very ill at the end. Please, this is NOT to say that Peanut is this bad, I am probably only trying to say that I understand how you feel and that only you really know how much both you and your cat can deal with.
Prayers are on the way that the tests come out with good results.