I've never removed wallpaper, but my understanding is that you have to put holes through the wallpaper with a tool called a "Paper Tiger" so that the water you put on the wall can get to the glue holding the paper on. Apparantly, lots of wallpapers are vinyl coated, and that prevents the penetration of the water to the glue.
Nowadays, no one uses real lime putty based plaster to repair plaster walls. Nowadays, everyone uses modern materials like drywall and drywall joint compound to repair plaster walls. I've been repairing gyproc lath plaster walls for almost 25 years now, and it's not difficult to do. Beg, borrow or steal a digital camera and open an account at any web picture hosting site (like w w w.photobucket.com) and take pictures of the problems you're facing and I'll walk you through the repairs.
And, this is true for drywall as well as plaster repairs, always always always do the repair with a bright light shining at a sharp angle to the area of the wall you're repairing. The sharp lighting angle will exagerate the roughness of the repair, giving you a much better idea of where you need to add joint compound and where you need to remove it (by sanding) to get the wall smooth. Once the wall (or ceiling) looks reasonably smooth under critical lighting, it'll look perfect under normal lighting.