Remove wallpaper that has been textured over
Removing wall paper   can be a major pain in the posterior  at best. With plaster on top it may be a nightmare, if doable. You could try it in a small test area.
It may be? easier and or less time consuming to skim coat a new layer of mud and sand smooth, but that requires skill that a DIY'er may not have (but with practice could do).
Maybe remove the dry wall and install new drywall, Again mudding and taping is an acquired skill, especially when you want a smooth non textured wall, it take a lot of prep to get smooth.