I'll add my 2 cents about using a trainer for rehab work. My in-home therapist came 2x week for about 4 weeks before I went to outpatient rehab. About the second week I asked about using the bike on a trainer (Wahoo Kickr) and she was completely supportive and we did the regular routine of rehab work then I got on the bike. I had raised the saddle to make it easier for the range of motion I had at the time and at first I just rocked back and forth with no intention or ability to do a full revolution. But, doing the back and forth rocking seemed to help and it gave me a focus mentally: I was back on the bike. It took some time to do a full revolution and the first one came unexpectedly: I kept rocking and rocking and then on day on one forward motion momentum just carried me over. It was a bit painful, but worth every bit of it.
The limiting factor for making progress in doing a full revolution was swelling. Ice and compression helped with that as well as doing enough PT to make progress but not so much as to cause more swelling. The first few weeks, as you very well know by now, are challenging because it hurts, it is swollen, sleeping is difficult, you wonder if everything is going to be ok, and progress can be slow and full of setbacks. But, progress will happen and your flexion and extension will improve and being on the bike, even just for rocking is useful work.
Keep up the good work!