Re: crooked wall
Forget the 2x4's.
Use a 1x and cut it to the same width as the wall, theoretically that is 4-1/2" +/- a 1/8". Now take trim with a 45* bevel on the top end and attach it on each side of the 1x to make a trough, then slip the trough over door jamb and plumb it up. Make a second trough for the other side. Now cut the head piece trim to fit the opening.
This method will allow you to size the opening however you like, yet maintain the look of a finished doorway. No drywall to fill in, no mess, other than finishing the new jamb and trim.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!