Sad to hear that you are in more pain and having more problems than before the 2nd ops.
The smoothening of the Patella has the consequence it is a new injury (although it seems to me it must have been there for some time, the roughening of the surface of thejoint). Any Patella problems will give problems going up/down stairs. The feeling of the knee having become more unstable has to do with the pain; your awareness of knee position is impaired due to pain and thus the muscles around the knee become less responsive. when you get less pain, when strength and coordination of the muscles improve with exercise(s) it should become less. Do realise that not only muscles directly related to the knee can have influence; the knee is part of the leg and if you would have other issues e.g. poor coordination of muscles around the hip this will influence as well. I suppose that when you 1st snapped your ACL this must have led to the wear and tear of the Patella. It takes a lot of time to do a good rehab, think of 1/2 year after you start full weight bearing. I suggest you get your 2nd opinion and ask for an exercise protocol. As I said before see to it that your whole leg is taken into account and not just the knee (when something goes wrong at the knee hipflexors, gluts, quads, hamstrings and possible calfs will influence it's functioning. Hope this will help.