Sounds like your post op rehabilitation wasn't optimal.
There are various programs here depending on the state of the supraspinatus tendon during repair but they usually involve passive abduction only and no anti gravity abduction for 6 weeks and other directions active-assisted for 3-4weeks at least after OP. The 8 weeks limit is not realistic here for ending treatment. Normally patients are on the way and doing active stuff but not there yet at 8 weeks. I don't have your OP notes or rehab program infront of me to study. It sounds as if physio could have been too agressive in light of the " slight posterior subluxation which the ortho said was due to a weak Wikipedia reference-linkrotator cuff ". The weak rotator cuff muscles will possibly remain weak and not hold the head of the humerus in the right place if their proper training is neglected and one does too much heavy resisted exercise with weights etc. The underlying weakness remains. The humerus joint can remain "un-centered" (sorry, we say "Nicht zentriert" here) and the luxation could get worse not better and cause more problems and pain. Sounds like that has happened.
My rotatorcuff repair patients do have pain at the start but it should slowly improve, even if not always at the same rate.
I in your position would get access to my records, operation protocols etc and get a new second opinion. Can you find another surgeon that does lots of shoulder operations? I'm not sure about a change in physio but try talking to the physio first. Agressive rehab and weights is probably not the answer at the moment. If the physio can't explain why she thinks things aren't working convincingly find another one. If I am not getting anywhere with a patient i consult a fellow physio and give the client further. Your shoulder needs a new good assessment from a good physio and a new start.
Hope this has helped you a bit!
regards