Can you wallpaper over existing wallpaper if it’s well adhered?

You can...maybe. Make sure to wash the existing wallpaper to remove any grease and dust. You have to make sure that you don't over-wet the new wallpaper. It is a good idea to try a small area first as the moisture will wick to the old paper and sometimes bubble. Instead of dunking the new wallpaper in water as usual, use a spray bottle just to moisten the glue side (pre-glued paper) enough to adhere to the old wallpaper. You may have to do small areas at a time for good drying. Good luck.

Yes, you can, although the previous edges may show through. It's not the ideal thing to do, especially if you know you'll want to change it in years to come. Each layer adds another problem for the person who finally decides they want to paint the wall, but then if you're not that person, no worries!

You can but it’s not the most ideal. If the first layer ever decides to release or peel, it’ll cause problems for the new layer. Even putting wet new paper over old could cause the old to warp and bubble. It’s best to remove it.

You can do it. But future homeowners will hate your forever. I removed 3 layers of wallpaper (plus 2 borders) in my kitchen and it took 6 months to remove it all. Let's just say that I wasn't happy with the previous owners.

Although it is 'possible' to do this, if you are still not convinced that it is unwise to attempt it, read through some of the postings here on Hometalk from folks who are now faced with removing multiple layers of wallpaper, or worse yet, having to fix a botched up job.