The issue about the guitar not sitting right in the case if you fit the button to the base of the heel is a red herring.

I did this mod to my Martin D28-S and had no problems, the case was shaped to accommodate just such an eventuality. Similarly with my Larrivee OOO-70, there is a recess in the lining of the case to accommodate a strap button fitted to the base of the heel, and I shall do the same modification there eventually.

I wouldn't know about cheap plastic or cardboard cases, maybe it would cause a problem ...

The balance takes a little getting used to, I agree, but, only a little, I have no problems whatsoever playing standing up.

Fat people might possibly find the balance issue problematical, and would be better with the strap attached to the headstock, even if it does look uncool.

As far as the strap slipping off the button is concerned, doesn't everybody use a strap-lock nowadays ?