Re: Sagging upper cabinets
The cabinets need to be fastened to the framing within the wall to keep them from sagging OR be designed and built to support themselves over that span. It was a careless and/or incompetent installer who would do otherwise.
You might be able to build a support frame behind the cabinets to carry them. If the cabinets are sturdy enough to be hung from the top, you could span the gap with three or four 2x4's laid flat over the depth of the cabinet. Another alternative would be to support the span from the bottom, though this will probably create clearance issues with the fridge/freezer.
The best option would be to have a custom cabinet made for the space that would span the opening and attach to the wall and adjoining cabinets on either side.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!