Inside edge cupping on the rear is probably due to misalignment. I think that the tire wear on those size tires will always be a problem, but the next set may do better with the camber kit installed. Good alignment doesn’t void the warranty.
Too bad mechanics don’t come with a warranty, for all that they cost the dealership.
I don’t believe you will find good performing tires in that size. They might do well when dry. Try to go cheap.