Rich,
I must have missed this great post when you made it.
Super analysis and pictures.
I've run into some of the problems noted; another is a receiver/frame combination that tolerances have just gone "wrong"- to the extent that the receiver will not slide on or if it does is too tight to move.
If the misfit is not too much, lapping will also correct the tolerance issue.
Of course you can have a combination of all of the above.
Thanks for posting.