Was the (mis)behaviour of some items somehow patched? I felt daring and put some unique weapons (Dawnbreaker and Harkon's sword) on Hearthfire plaques and they haven't fallen off yet, nor did they move or grow into the wall.
I didn't experience the mannequin dupe glitch thus far, either.
Not that I know of. Personally I have never experienced any item misbehavior with plaques; for me it's the racks that mess up. All four items in the racks in my Lakeview entryway are irretrievable. Luckily it's stuff I would never want to use. And the mannequin dupe glitch still works for me. The other day I took my Ancient Falmer Armor off one of the mannequins in my Armory to give to Faendal. I didn't even leave the room before it was back; I just turned around to put something else on the mannequin and the armor was there.
Hey there, sweetroll; I just cast Oakflesh... if you know what I mean.