Welcome to my world nothing is square, plumb, or level it's my job to give it the appearance of square, plumb and level. As for the uneven drywall I don't know if this will help or not. There is a company called Trim Tex that makes a product called "shim on a roll" it's the same width as 2x materials. It comes on a 100' roll in 1/8 and 1/16" thickness and is perforated every 18" I have found it great for squaring walls for drywall. It runs around $10 a roll. Hope this helps.