Regular mud for first application and 'lite' for the next ones.
Durabond 45 or 90.
Sheetrock is primarily gypsum, combined with sand and water.
Drywall mud is the thick, goopy white stuff you use to fill the seams between boards.
when you hang drywall, you have to tape off the seams with paper tape and "mud", then "float" the mud smooth. the drywall float is a flat metal tool on a handle that you drag across the mud to smooth it out, like when you smooth out the icing on a cake
Nothing
I use basic drywall primer over all of those.
lightly wipe the drywall mud after it drys
If it's small spread mesh tape over it, then mud it. If bigger than doorknob size, put a patch of drywall into it then mesh tape and mud.
They install drywall, sometimes tape and mud it also, although commercially that is a more specialised job.
No, you plaster or mud it first.
Usually, there is drywall under the wallpaper. It is not difficult to remove the wallpaper (by steaming, or another method) to get to the basic drywall. Then, you can texture it and then paint it.
Yes it can if you stir it well.
You take a solution of drywall mud and a texturizing tool and apply the mud very thinly in the desired design/texture that you are looking to get. There are a variety of tools available to do this job depending on the type of texture you want. This may be a job best left to a professional. Call some local drywall companies and get at least three estimates from qualified contractors.