I'll drum up some activity on this forum by providing yet another update, especially to my last post!
In the end, I received two new bodies and rear bumpers. I test fit and interchanged parts and I've determined the problem has to do with the body. That's a different conclusion from my last post, I know. I still can't visualize what's wrong with the body, since it doesn't look visibly warped, but somehow the trunk lid edge sags too low compared to the sides, causing the bumper to ride up. At least now I have two kits with proper fitting bodies and bumpers.