Try some form of oxy chemical to remove.

First off don't let anyone tell you that you can use a chemical stripper.

But this all depends on if the tenant helps out and doesn't bring more bedbugs back into the unit.The idea behind the chemical treatments is that the first will kill the adults, but not the eggs.

My advice is not to waste your money on testing clean with bleach water and buy a mildew mold suppressant from hardware store it's a chemical you put in a pump up sprayer keeps mold from coming back if on a painted surface get a fungicide paint additive and repaint area.

I often wonder if they were ever affected by the 'bomb' chemical residues as they apparently never bothered to steam clean the carpets or even have the vents cleaned.I recently saw the home again after the current owner gutted the lathe & plaster to upgrade the insulation, electrical etc.

The owners wife told me that No Chemicals or Haz-Mat type of things were ever mfg'd there .

The GC seems confident that gutting the place and replacing the roof asap would immediately help problem & then obviously treat with chemicals etc.. its just hard to know 

Also, Is it realistic to ask housekeep to keep up with chemicals?

Typically, removing smells from drywall/sheetrock requires either complete removal and replacement or the use of stain blocker primer paints or other chemicals.

There are also wallpaper removal chemicals from Home Depot.