What exactly is "behind dpkg's back" here? This was discussed, in the open, years ago!

This was implemented, as an option, years ago. This was implemented fully in other distributions years ago! Fedora has had it for a decade, with few problems.

Dpkg has a few minor bugs with it so it needs to be fixed. It's holding up progress here.

In that usrmerge adds symlinks that dpkg does not know about, doesn't manage and doesn't understand. Like if the sysadmin added random symlinks in various places. All bets are off after that. I'm surprised the amount of breakage isn't higher TBH.