Hi and thanks in advance for any help.

Background: I have an older home from 1943 and the plaster wall is ½" out of level from right to left in my kitchen. The marble slab backsplash will run the full counter length (132") in three sections (slab will be seamed). I've installed the upper cabinets and they are shimmed from 1/4" to 3/4" to keep them level with the bulkhead above the cabinets and against the uneven plaster wall.

Question 1: Should I attach 1/4" sheetrock to the wall and shim it out right to left to provide a level surface for the marble to be attached?

Question 2: Can I run electrical behind the sheetrock for outlets?

Question 3: The slab is 1 1/4" thick...can the outlet receptacles be proud of the sheetrock an inch?

OR, should I just route slots in the plaster (plaster is about 1" thick) and run electrical through it? There is cinder block behind the plaster to make things more difficult...This would have the backsplash ½" out of level right to left but perhaps that's not an issue? My OCD is triggering at the thought of this...

Thanks again!