If wallpaper was painted over you could tell pretty easily by pulling on some of the peeling paint and breaking the paint chips. Wear a respirator mask while doing this, however, as often times flaking paint is a potential indicator of lead based paint. If the chips contain paper, then you're right, it's wallpaper with paint over it. If it's just paint, then be more careful - get the chips tested for lead.
Given the wide spread flaking, it is likely that the wall simply wasn't properly primed. An improperly treated wall when painted over will eventually lose adhesion with the paint and it'll flake away like you're seeing. If they applied paint directly to wall paper w/o priming, I think the same is true.
That 2nd picture DOES remind me of wallpaper... I've scrubbed far too much backing off the walls and that looks similar.
The grey subsurface is, I think, a kind of stucco mix that was often used to even up walls where lathe and plaster was replaced with the older style 2x4 drywall panels. It's nasty, gritty, dusty, unpleasant stuff, tougher than joint compound/plaster to work with because of it's tendency to crack and break rather catastrophically. When I run into that stuff in my rentals my approach is, "IF I have to touch it at all, it's ALL coming down." Plus with wallpaper I swear gutting is easier than stripping.
Now around the vent pipe, that looks like moisture damage. The bubbling around the pipe suggest water leakage. Is that a "finished" ceiling - ie - thats the roof on the other side of that wall w/ the pipe? If so, make sure it's properly sealed and replace at least that area of ceiling.
Picture #5 seems to confirm this - someone touched it, and patched it badly.
You can drywall over the plaster, but you'll have to do the entire wall and screw through the plaster into the lath/studs.
Alternatively, you can fix the plaster itself. The problem is, it's separated from the lath. The fix for this is to drill shallow holes into the affected area JUST DEEP ENOUGH to penetrate the plaster, not the lath. Drill the holes 4" apart in a grid pattern.
Then use a caulking gun and inject construction adhesive into each hole to create a glue layer behind the plaster between it and the lath.
Press the plaster up against the lath with a flat board, holding it ONLY UNTIL THE PLASTER STICKS. Don't leave the board there too long or the glue will grab it too. Slide the board up and down to keep it from sticking while the glue takes hold between the lath and plaster.
Allow it time to dry, then patch the holes and paint.
Best Answer
That crack is most likely directly inline with a seam between two sections of the underlying drywall. Environmental effects or settling of the building structure over years can lead to cracks like this opening up.
Probably the best fix for this is to sand to roughen up the area for six to eight inches or so on either side of the crack. Then apply some good quality mesh style drywall joint tape:
(I would actually install in two overlapping layers).
Then use a drywall mud knife to embed the tape into a layer of drywall mud that is feathered out over a wide area on either side of the mesh tape. If the drywall mud is installed carefully over multiple applications you can minimize the amount of sanding required.