Here’s a new one — I did as you described, got my background all panned and zoomed and keyframed and happy, and then hit “break apart clip items” to do my opacity keyframes. As soon as I selected that, I lost all the previous movement. The clip went to a large default zoomed-way-in view.
I’m now going to try it the other way around and see if that works better: break it apart first, then do the opacity keyframes, then convert it into the compound clips for the panning & zooming. Will report.