The tailbone pain should be investigated. I would pursue it until you have an answer. I do not believe the position of the body on the OR table caused this. The surgery position usually affects the lower back, not the tailbone. The inability to sit in one position for more than 5 minutes is not normal.
The knee pain is normal from prednisone use and may or may not go away in time. Seven years is a long time to be on pred and I think as you gather, you know there will be some long term damage for that. You might be able to lessen the pain by hiring a trainer and doing some specific training to build up the muscles around the knee.