Another excellent summary, thank you!
Well, I would suggest that the 'lower end' actually does stop around 48 but it can be off set by various other considerations such as length of the condition, for example, a patient with rheumatoid arthritis might be operated on at a much earlier stage as the prognosis for their general health is poor.
However, your situation has been precipitated by the amount of damage from the torn cruciates and subsequent surgery (the xrays were very helpful, thank you). It is an almost inevitable consequence of such surgery, I am afraid.
The Fulkerson osteotomy I knew as an Elmslie and is for a transposition of the tibial tubercle on which the patellar tendon is fixed. Now this is interesting as this procedure is usually carried out for teenagers with a laterally displaced tubercle and therefore tendon. The patellar tendon is a principle support structure for the stability of the knee joint so the fixator - the tibial tubercle - is moved over the increase the tension. Much like you would move a tent peg to increase the tension on a guy rope to tighten a tent. It does, of course, also shift the weight bearing to a different area of the joint surfaces. This might be in concert with your physios suspicions that you have EDS though I doubt that, overall, it has much bearing on the condition of the knee joint itself at the present time.
But as to what can be done now - well, IMVHO, a TKR would most certainly not be an option at your age. Have you asked your surgeon about having arthroscopic washouts? This would rid you of all the debris that is cause much of the scrunching and pain and enhance the quality of your life. At worst it cannot make things worse. Losing weight would almost certainly help as well though I suspect you have already been advised of this. However I would urge you to set this as a priority in your situation as you are now battling for your quality of life and whilst surgery is not an option for you for maybe the next 14-16 years, a loss of as much weight as you can would very much be your salvation just now. It's a tough break for you as the inactivity caused by your condition is the very thing that causes the weight gain and vice versa. Your actual catch 22 situation.
I wish you all the best and only wish I had something more encouraging to tell you. Having gone though all that it seems hardly fair that you are faced with such a rotten result. I wish you all the best. Please come back and chat sometime if it would help.