So getting back to your actual question
I like my idea of essentially making a box under the counter and using the plywood sides to support the shelving.
Jon in Az's idea of furring strips tabbed into the fg would be strong but could strain the fg depending on your design. Furring strips could also be screwed to the fg from the outside but then the screw heads would be visible. Carriage bolts would also work and would provide a cleaner appearance from the outside.
Fred and David's idea of using the wheel well would provide great support - just make sure any screws in the wheel well area come from the outside in.
Personally I would ditch the mousehole
electrical entrance for a sealed unit like a furrion style. That will keep the critters and the elements out and free up some room but then you have to store your electrical cable somewhere else.
The rest of the plumbing and electrical can be neatened up and kept at the back - the only time you'll see it is when the cupboard door is open anyway.
Hope that helps.