Re: Sagging upper cabinets
I think that adding a strong-back or two to the cabinet boxes might make them into an assembly capable of supporting themselves and the contents.
I have to assume that they have faceframes; The bottom chord will be in tension, so letting in a length of threaded rod with washers and nuts at the ends will prevent the bottoms from spreading. The rod should be as close as possible to the floor of the cabinets, and the washers must be as large (in area) as will fit.
The top of the cabinet assembly is in compression, but must be strengthened against flexing downward. Steel plate or angle will do for the task, but a few slits will need to be made in the box's sides to admit the angle. Then plenty of screws (#12x3/4") to attach it.
These two different steel elements will unite the two boxes into a structural unit.
I would do one more thing: build out some framing behind the cabinets to fill the 5" air space and give some fastening for the cabinets to the wall. It's unconscionable that the installers did not even do this.
It was wishful thinking to mount it as they did.
Remove not the ancient landmark, which your fathers have set.