I have an old house built in 1849. I am trying to redo 3 rooms and have run into some issues. First the paper in the Dining room came off great with a steamer. However i am left with gray sandy plaster walls. It looks like these old Yankees never put on a finish coat. Should I just wallpaper right onto the gritty plaster as they did before, or paint it with primer first

Second issue is the living room. It is plaster and may places of repaired wall with drywall. I 1st coat of wallpaper came off fine, but now I am left with a painted surface with lots of glue. The paint is in good shape. If I am going to wallpaper do I have to take off the glue? Should I primer before I put up new wallpaper. So confused I don't know where to start.