This may not be much help in analysing your right knee, but I hope it reassures you that you are not alone in having symptoms that remain a mystery after investigation. I have had problems with both knees for several years and 15 months ago, I woke up one day unable to walk because my knee was locked and the size of a football. I went to my local casualty and they referred me to the next day's fracture clinic where the consultant could not understand how my knee was (in his words) so angry, but without any single event/accident to cause it. I had an arthroscopy 3 weeks later, which found some wear and tear, in keeping with my age, but nothing else. The pain and swelling continued, especially after exercise and an MRI scan revealed nothing as well. I was having regular physiotherapy and the physio was very supportive, even writing to the consultant because I was convinced I would be seen as a fraud. I was then referred to a second consultant who did a further arthroscopy 2 weeks ago. The wear and tear has been smoothed off, but my knee is still sore, and I'm hoping it settles soon. Apart from that, all my ligaments/tendons and cartliages are in good condition so the level of pain I have had is a mystery. My other knee is also troublesome and I am on the list for an arthroscopy of that one. X-rays suggest there is little, if any, arthritis in that knee so I may find there is no answer. The physiotherapist who reassured me told me that many people have knee problems that have no apparent cause, and therefore I should not worry - that was good to hear as I have found generally, NHS staff are overworked chasing targets, and unable to provide proper advice and sufficient time for patients. All I can say is keep telling them how your knee is.
Hope this is of some help.