I know this isn't the answer you want to hear, but I don't think priming and painting will hide the tape.
Before you apply any more primer, I'd add another thicker coat of mud. Slather on the mud thick first - it doesn't have to be smooth. Then smooth it out by angling your 12" knife so it's almost parallel with the drywall as you drag it along the surface. You'll have to press fairly firmly in order to avoid air gaps appearing in the thick coat you're smoothing out.
You may have to get a feel for it. If you push too hard and can see the tape again, redo that section until you get it just right.
Wait for the thicker coat to dry, then sand the edges as smooth as you can get them. Very carefully/lightly sand over area covering the tape if you didn't get it quite smooth. You may need to do a second coat along both sides of the thicker coat to make it join the drywall surface smoothly.
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.
Best Answer
As already mentioned, I would use a piece of Sheetrock to fill in. I would add a couple of points:
You'll want to use mesh tape on this. Plaster is portland cement based, meaning it likes to absorb water. The mesh tape will hold up better compared to paper tape here. I'd also look for the rot-resistant sheetrock for the same reason.
Secure or remove the loose plaster behind the sheetrock before patching. Ideally, it would be best to get the sheetrock to rest only on the lath. I'd even run a bead of construction adhesive between the lath and sheetrock (along with screws).
For patching plaster, I've used the following procedure with very good success. This assumes good attachment to the lath. If the plaster keys are broken/loose, then that requires some more attention. Crack repair also needs dug out before repairing.
Remove any loose or soft plaster pieces. Vacuum out the area to be repaired. Use something like Elmer's Glue-All and mix 50/50 with water. Using a brush, apply the glue mixture to the old plaster that will be repaired. Again, the reason for this is the plaster likes to absorb water. The glue mixture will help seal the old plaster so that it doesn't take the water from the repair material and also aid in adhesion. Mix up some Durabond 90 for the base patch. Durabond is very strong but it is not intended to be sanded so make sure it is not proud of the finished surface. As a side note, Durabond or other drywall compound that comes in powder form is (almost?) always a setting compound. This means is actually cures like concrete rather than simply dry out like the pre-mixed joint compound. I think it's generally understood that setting compound is stronger than pre-mix. I recommended Durabond 90 as the 90 means it sets in 90 minutes. The longer the set time, the stronger it usually is. Once the Durabond is mixed, try to apply it to the repair area while the glue/water mix is still a little tacky. Allow this to set before applying "sandable" joint compound.