It comes in a spray bottle and is designed to stop/prevent/kill mold and mildew

They work better than spraying and 'sealing' treatments.

The popcorn is not there primarily for decoration or to deaden sound from above, although there was a time (late 70s, early 80s) when a lot of people did think that a sprayed-on popcorn ceiling finish was the classy thing to do in your house, along with swag carpeting, puke-green toilets, whole-wall murals, painted representations of wood down the sides of your station wagon, and those forever leaking waterbeds.So when you commit to scraping off a popcorn ceiling, you have to accept that you're probably going to find an additional problem under it.

They cut out two holes (one is maybe 2x4 and one is maybe 1x1) to dry it out and sprayed chemicals to prevent mold, and then the holes were patched up and ceiling repainted. 

You can treat it if it's on certain surfaces with a spray but my guess is you're dealing with something much more serious then that if it's been uninhabited for a couple years.  

If your only problem is  "difficult to get key in all the way and back out" spray some wd40 and that should solve your problem

Added solar panels and wall to wall renovations along with spray foam insulation.

For townhouse, we will treat with the whole-house pest spray before move in, then leave a bottle of that, plus Ant traps & hornet spray with the home. 

However, for a handyman to spend 45 min walking the cutters and spraying them once a month in the interim costs me $20.

To spray roundup on a standing crop beyond the label causes harm to the crop, and is breaking federal and state law.