They are the same.
Those are different names for the same thing.
Sheet rock 90 is a type of drywall joint compound that is specifically formulated for embedding paper tape on drywall joints. It is designed to provide a smooth and strong finish when finishing drywall seams during the taping and mudding process.
Sheet rock mud.
Yes. Be sure to use adhesive between the layers. The screws you should use are called laminating screws. They are slightly larger than the regular drywall screws (Number 9 versus number 8 or 6). Be sure the drywall you intend to cover is in good shape and secure. You don't want both layers falling off the wall.
The best sound proofing drywall plywood and adhesive solutions are somewhat a matter of debate. Some of the top selling solutions include sheet rock with SilentFX or QuietRock.
Drywall is commonly calcium sulfate dihydrate, or gypsum. Nowadays there are other types, notably flue gas desulfurization (FGD) drywall and 'Lite' drywall containg ash.
Yes. 'SheetRock' is a particular brand name for gypsum board. I believe 'Drywall is, too.
Drywall is usually made with gypsum.
The standard thickness of drywall used in commercial applications is typically 5/8 inch. It provides better fire resistance and soundproofing compared to thinner drywall options.
The main, and only rock in drywall is calcium sulphate dihydrate. (colloquially known as 'gypsum' )
Gypsum
I have seen a crew do it in one day for $700 here in CT. (and they were working with fire code sheet rock). This was strictly to hang it, taping was not included.