Installing door in 2x6 wall (drywall only on 1 side)
For #3, I'd buy a prehung door with a 6-1/2" jamb (normal for 2x6 construction) and set the jamb 1/2" proud into the workshop. This makes it easy to add drywall to the workshop in the future if desired. (And personally, I'd skip casing entirely since you're at bare studs in the workshop. If you had to have casing, I'd butt it against the jamb instead of overlap it.) 

Comment with any questions...