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Tricky to estimate how filthy it will be just to get down to the plaster. Personally I couldn't leave the wallpaper on and any method that gets that paper off damages the plaster underneath so you have the repairs to that then I would put lining paper on it because it's a hallway.
Massive job whichever way you look at it.
Like I said though, builders would knock it all off and render or pb. I have done work like this in a more decorative fashion but you have to ask yourself by the end whether it's worth it. Of course it's a decision that you wouldn't make based on a couple of internet photos.
If the original lime plaster isn’t blown and the surface can be stabilised there’s no reason it can’t be skimmed over.
Knocking it all off is a filthy job.