You could possibly cut out a fairly large section of drywall. Leave enough at the top to patch without disrupting crown. Then, using a string line and a spacer block the same thickness as your drywall, sister new studs to the old ones and place them where they need to go to create a flat wall. Then attach new drywall to the new studs. I used this method recently to create a flat wall where there had been pretty severe termite damage to the edges of studs. It worked well, but I didnt have to worry about saving crown molding.