I like your theories and would like to add some additional ones of my own.

I was diagnosed with seronegative inflammatory arthritis approximately 20 years before PMR was diagnosed. PMR and inflammatory arthritis can coexist. The two entities cause inflammation in structures that are in close proximity to each other.

Synovitis is one of the hallmarks of both PMR and inflammatory arthritis.
https://creakyjoints.org/education/what-is-synovitis/#:~:text=In%20RA%2C%20your%20immune%20system,Synovitis%20equals%20active%20inflammatory%20arthritis.%E2%80%9D
I would guess that PMR exacerbates arthritis.

The link between prednisone and arthritis is clear. Prednisone is known to have musculoskeletal adverse effects.
https://pubmed.ncbi.nlm.nih.gov/1429389/
and
https://pubmed.ncbi.nlm.nih.gov/12025801/
I would guess that prednisone exacerbates arthritis too.

Lastly, I would guess that the damage caused by inflammatory arthritis and osteoarthritis all begins to look the same as we age. The wear and tear arthritis blends into the inflammatory arthritis so that it becomes hard to tell what causes what.

I don't think inflammation is confined to any particular area of the body simply because we happen to be diagnosed with PMR. I think of inflammation as being able to attack any part of the body that it chooses to attack.

After years of elevated inflammation markers and a myriad of problems, my rheumatologist stopped saying that I had any specific diagnosis. She said I was on prednisone to help regulate "systemic inflammation."

All I know is that I'm "muchbetternow" since getting off prednisone