the 28" rule is a bit wonky, I'll agree - especially the marble concept. "falls" or "veers downhill"? Many cars cars have a smooth flow from "roof" to rear glass (integra, S14, RSX). the MR2, del sol, X1/9, proper hatchbacks and wagons and pickup trucks have a pretty sudden drop off, though that is at various points along the length of the car. the notchback term is very subjective, as is the 28" method of determination (and I'll even offer that a reasonable definition would hold that the end of the roof is a point of, or within a small region of, inflection from the highest region of the car to that which connects it to the rest of the vehicle, but I know that's not going to be universally accepted)

as for the bumper = body: IT rules would include an integrated bumper (all cars today) and exclude a separate bumper (early A2 VW, E30, etc...) from the "body". the GCR definition uses "licked by the airstream" and I'll argue the bumper is that in either case.

I'd think the MR2 would NOT be a notchback, but it is an interesting question.