Not always. If you are putting the panels full 8ft high you probably need to secure them with adhesive. You would have to experiment to see if your wallpaper is secure enough to allow this. However, if you are doing wainscot height and use the base and cap that is part of this system they should secure the panels. Good luck!
The molding kit is sold separately, Im using it right now and I love it. It is very easy to work with and easy to install.
This product expands and contracts too much for that big of a space.
There is absolutely no wood-grain, they are smooth. 7.25" includes the tongue and groove. Yes - Six pieces covers about 42" Subjectively, these are a fantastic investment and I am very happy I decided to go with them. I tried the MDF versions (awful) and ended up happily choosing these.
Yes it is.
You would have to prime it with primer designed for best adhesion of your paint.
Inside corners could be trimmed using a vinyl cove mold which should be available at your Home Depot store.
Panels cut easily. Best way would be a power miter saw but could also be cut with a circular saw however a guide would be recommended.
Not sure what you mean by wood trim. If you mean wood base molding, no - these panels fit into a slot in the top of the base molding (bottom) and mid molding (top) to hold the panels in place. Existing molding wouldn't have the slot.