Nothing respawns, the mannequins still make free armor, display cases work the same as the Hearthfire ones and all the storage is safe. You cannot get rid of two of the skulls in the third display case.
you can forget using enchanted weapons in the weapon racks...
I have enchanted weapons in the weapon racks? They are just as safe as any other weapon racks. If you have trouble with your enchanted weapons it is possibly because the display coding was altered when they fell through a display case. Some Orc weapons come with demented display code, and keening and a few other unique weapons refuse to behave with weapon racks and plaques. That is not the fault of the weapon rack. if a weapon does not display properly in one house, it will not in any house.
I put two of the new Dragon Priest masks on one of my mannequins and then put all Miraak's gear over top of that, thinking I could use the mannequin for armor storage like any other mannequin in vanilla Skyrim, but sometime later my masks disappeared. I'm glad it was the masks and not Miraak's mask, although admittedly Miraak's mask was far worse than both, but still.
Just be careful not to put excess gear on the mannequins and you should be fine.
You do not use mannequins to store stuff, there are plenty of containers for that.