What's the proper way to install veneer/cultured stone where sheetrock is currently at? Can you wrap the sheetrock with tarpaper, metal lath, scratch coat then veener. Or do you have to remove the portion of sheetrock, install plywood, tarpaper, metal lath and scratch coat?

I recently purchased a fairly new home (2004). I was in the process of removing a wallpaper border in one of the guest bathrooms when I found mildew on the drywall under the wallpaper. The border is difficult to remove so I assume the previous homeowners did not size the walls, just applied it to the painted surface. While removing it some of the paint bubbled up (I guess due to the inproper application and the removal solution) and there is some mildew under the paint below the border. The drywall is not spongy or crumbling, there is no mildew smell, and no visible mildew, it's underneath the paint and border. I am hoping the problem is something I can solve myself. I did notice some "shadows" on the walls, but they don't look like the water stains I googled. They look more like dirt, although I assume it is the mildew as it would be hard to get dirt that high up. There is an exhaust fan which I plan to use religiously but I don't know if it was used before. Also, should I buy one of those moisture testing devices and check the drywall even if it appears to be solid? The area I noticed the mildew on was directly above the shower enclosure. I have not removed all the wallpaper border yet as I was too upset upon finding this, but I have to do it very soon. My question is do I have to remove the paint so I can get to the drywall underneath, or can I use a bleach solution directly on the painted drywall to kill the mildew? If I have to remove the paint to get to the drywall, how the heck do I do that? Any help would be greatly appreciated. Thanks.