This seems like too high of a level of pessimism given that the usrmerge
package already implements this sort of force-merge and some people have
it installed and don't seem to be running into a bunch of bugs. The last
round of problems was on systems without it installed, because packages
generated that way weren't backward-compatible, not with the forward
direction of forcing the merge.
That said, if we do want to go down this path, getting as many people as
possible to install usrmerge now and make sure it doesn't break anything
(or report bugs if it does) seems like a good idea. (Just don't build
Debian packages for upload on the resulting system; use a build chroot
instead.)