How to prepare a wall with drywall for painting after wallpaper removal
Skim coat is more reliable than magic paint, but no direct experience with that particular flavor of magic paint, just seen lots of problems in general with magic paint and wallpaper residue.

The counterintuitive (at first) fastest (and thus cheapest when labor is a cost, however you count that cost) approach that's 100% reliable is to rock over it (you can use the thin stuff), or rip and replace the rock. Skim coating seems like it's less drastic, but can actually be a lot slower.