How to seal and paint a rough plaster and brick wall

You do need to seal the plaster otherwise the new paint won't stick as you point out.

My first choice would be to wipe down and then paint with a coat of diluted emulsion as you suggest. You might want to try this on a section of the wall before doing the whole lot to see if you get a good finish.

Dilute PVA will definitely seal the plaster and I can't think of an obvious reason why you shouldn't use the wood glue - but there may be differences in the formulation you need to look out for. How dilute did you make the mixture in the past? I've used a 5:1 water to glue ratio for sealing a ceiling I was going to paper which worked well. Again you could try a higher ratio to see if you can find a mixture that still seals but then accepts paint better.

You might be better off sealing with the dilute PVA then putting up lining paper - which does stick quite well - and then painting that. The benefit here is that the lining paper will smooth out the wall further hiding any remaining imperfections.

Of course the solution that will give you the best finish is to get another skim coat of plaster applied.

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.