how to properly hang door when wall is far out of plumb?
Depending the style of trim and width, will help determine how much fudging can be done. Usually, depending on the trim choice and whether it is mitered or not, 1/2" can be taken away by moving the jamb past the wall at the top and the bottom to aid in getting it closer to plumb and carve away the drywall strategically to help ease the trim to meet the wall properly.

What you mention in your edit is worth another idea. Since you are willing to feather it out to be plumb, or at least more plumb, you could remove enough drywall to allow shims to be added or sister studs to the existing to get the wall plumb. Perhaps it could be done on the other side of the wall as well? It will require a wider jamb, but it will be plumb...