Many apologies to ask the question and vanish.
The gist of it was that I had to open a case with the mother ship and go through at least three techs...basically re-stating and showing them the problem...until finally, magically, someone "up there" cleared the reference to the in-place holds so that the user was able to finally empty their Deleted Items. Probably ran adsiedit against Azure AD, I would assume.
I had tried clearing the value in the on-prem AD and synching and the value would always return.
Of course, MS never bothered to share with me what or how they did it, which is a real shame. Even if I don't have sufficient rights in their cloud to do it, the old-time Exchange Admin in me was really curious. But, alas...
So, go ahead and open a ticket and prepare for a few days of relaying until the right person gets the case.