I've tried the frosting in a spray can that is supposedly meant for a bathroom, but it was worthless.

perhaps a huge bead of spray foam along the ends of the joists and the first few feet of wall would be a big help?

Instead, I used a similar cleaning product called Spray 'N Forget.

Will it be necessary to treat both sides of infected drywall or is there a spray/powder which can be put in behind the drywall?

Even if you pave the entire thing in concrete, someone is going to have to spray the weeds that come up in the cracks.

I flip a switch, hit a reset button, or spray some graphite and we are good to go.

I like to spray with Brush-be-Gone or Roundup and then spray any new sprouts during the growing season.

I ripped down some of the sheetrock (behind the tub spout)  and saw there was some minor mold growth so I sprayed some concrobium.

If you are comfortable doing this yourself and it is legal in your municipality, look up pest control products on:http://www.domyownpestcontrol.com/Suspend SC is a good one and has a long residual of several months, which means it keeps working for months.Pest control companies use this product for general pest control and they typically spray along the entire outside walls (bottom part) of the house and all of the baseboards inside the house. 

Eventually we got a sprayed-foam roof and the problems were over.