My OS also won't do an injection into the joint. When I had continuing problems with tendons/soft tissue, he did do a cortisone injection around the biceps femoris tendon. That had a short-term good effect, so I had an ultrasound guided cortisone injection in another area around the tendon that was too close to the nerve to do in his office. Again, a short-term good effect. I also wore a brace which prohibited full extension (to relieve the stress on the tendon) and had additional PT...which didn't help at all. The next step was a tendon needling procedure right into the tendon which completely eliminated the pain for about a month. Since then, the discomfort in that area is minimal and I haven't needed any more treatments in that area. I continue to have soft tissue discomfort around the knee and use lidocaine patches as needed.
Even after two years, my knee gets stiff (although it doesn't sound as bad as yours). Every day I ride my recumbent bike for 10 minutes, which helps to keep it moving. I have about 110 flexion as long as I keep it bending. If I don't keep working on it, it will go back to 90 flexion really quickly.
I was also 49 when I had my TKR (followed by a MUA), so I understand when you say the surgery hasn't given you the knee you expected. While I have gradually progressed to being very happy with my new knee, it still gives me enough trouble that a 20 minute walk makes me limp. However, I'm not in daily pain anymore and truly do feel like I have my life back. If you are unhappy with the (non)answers you are getting from your medical team, I think a second opinion is always in order. Although your OS might be "the best in town," a new pair of eyes may see something that he is missing. Try to find a surgeon who only does replacements and sees patients with "failed" TKRs. If you don't know where to start, a university teaching hospital is a good place. Or go to kneesociety.org and search for an OS in your state. This is a select group of replacement surgeons who are studying and researching.