It's hard to say without knowing what the exact "type" of HD it is. Slipping in and out of the socket because of what? I forget all of the terms but I believe the acetabulum is the (socket) and the Femoral head is the ball.
Surgery is expensive, but if her situation would allow for a TPO they reccomend that it is done before a year, or shortly after depending on the wear and tear of the joint. The less wear, the better the outcome. Otherwise the only other option is a THR (Total Hip Replacement). A FHO (femoral head osteotomy) is usually done on smaller dogs, but lately it's been done on larger animals. It's not the preferred option for dogs as large as Rotties.
Although something about what your vet said contradicts what I've been told from many who's dogs have undergone a THR. The majority state that it's the best decision they have ever made, even though post-surgery is hard, but they would do it again in a heartbeat.
TPO post-surgery stories are even better.
I have heard some horror stories, but most are because post-surgery the proper precautions were not taken. The first 6 months after surgery are hard, but 6 months compared to a lifetime? Definitely worth it.
I don't want to worry you seeing as I don't know anything about your dogs symptoms, but it does sound as though you've tried a lot and she is still in pain.
If I were in your situation, (which I was a few months ago), I would get her x-rayed and send them off to a specialist. They do the surgeries everyday and will be able to tell you if she does need surgery and if so, what kind.
I know how difficult it must be to see your pup in pain. Keep us updated and good luck
