Go buy a steamer, if you don't already have one, from one of the big box stores and start taking that wallpaper down. Feel along the walls first, top perimeter, bottom perimeter, and everything in between. If your plaster is in horrible shape behind all the paint and paper, too the point that it cannot be saved, you are gonna know from feeling along the walls. If the walls are sound, then start taking all that paper down. I am going to assume that the entire walls have at least one layer of wallpaper that has been painted over with a border hung. Start the steamer up and go to town. Painted over paper can be a pain in the a$$ to remove. Be patient, and do NOT try any of the enzyme crap. The steamer is the best way to go. The rooms that I have done in my house had 5 layers of wallpaper with 5 borders along the top, which equals out to 10 layers of paper along the top perimeter of the rooms. One room had paint over half the paper. I had bubbling, and cracking of the wallpaper/borders along the top of the room. So, I started at the top. Put the steamer on, hold it firmly against wall for 20 seconds (or longer if needed), move it over one paddle width and keep there for 20 seconds. While your on the second spot, scrape the paper from the spot where you just took the paddle off. It should come off pretty easy. Just keep doing the same thing over and over till its all done. It will be super messy though. Have plastic taped to your baseboards and spread out far onto the floor to catch the water that runs down the walls, and all the paper too of course. When all the paper is off I prefer to quickly run a drywall screen over all the walls to remove any remaining glue. Then patch and repair the plaster if needed and prime and paint.
If you have a wallpaper border only, with painted walls, your job just got a whole lot easier. Just do the same thing as above along the border, then sand it smooth along the line where the paint/border met, prime,paint, voila!
Old paper will always bubble and crack over time, especially if it has been painted and/or has multiple layers of paper. It does not necessarily mean the plaster is in bad shape. I know it sounds like alot of work, but will be totally worth it in the end. It will not be a can of worms unless you have some serious issues with your plaster, which is why I suggest to feel the walls first.
If it does sound like too much work for ya, just paint the walls and put really dim light bulbs in your hallway fixtures :D

I would definitely open that can of warms without even thinking. The worse case scenario is that you would need to remove some of the old plaster and replace with drywall. No rough plumbing or electrical wiring involved..... LOL.
When it comes to house remodeling plaster is one of the cheaper fixes. You can find a friend that might be willing to help put the drywall or hire a handy man. It is an "easy" job and shouldn't cost you that much. However, you might not need to do the drywall at all. You might just need a coat of plaster on top just to smooth it. It is even cheaper. Or you could decide to just buy a paintable texture border, cover what you need to cover and paint over.

I think you would love it, feel accomplished and learn a lot. If you are inclined to become a DYI folk that is absolutely the best project to start with....

Good luck!

6 inches is a regular drywall knife, and very good for wallpaper.

I hope you are TAKING PICTURES of the BEFORE, because you want to have bragging rights when you finish. Anyone who owns a centenarian house like you do, will discover a lot of challenges as you poke around. I do so envy you the opportunities you will have to bring this house alive again.