If I am reading this right, your quilt shifts inside the duvet cover, no matter what you have done. I can't, however, tell if you have tried pinning all the way through all layers. I would think that would work, say every ten inches to keep it in place, like patchsamkin suggested. If the pins (safety pins) held it in place, then I would get some strong thread and large needle and sew several times where each pin is.
I have only tried using a duvet cover once, and it fit my quilt, but it still shifted on me. There is just something about a duvet cover that I didn't quite catch, I guess. In which case, I haven't been a bit of help, huh?
At any rate, I wish you success. You will find more suggestions to come, I am sure.
Dina