Drywall over crooked/unplumb framing
We bought an older home that, among other issues, doesn't have a firewall b/w the attached garage and living space. The previous owners laid 1/2" drywall from bottom to top plate and stopped, leaving the area from top plate to roof open, which goes into the home envelope over living space (and also houses a high-pressure HVAC up there. they also didn't seal the sill plates, etc). I'm often guilty of shooting before I aim, so I ran out, grabbed 10 sheets of 5/8" type X and started hanging! As I haven't decided if I'll ever put a ceiling on it, and the fact that I'm hanging solo - I started in the front left bottom corner and hung them vertical. The first couple sheets fit tight to the corner, and were plumb - then I noticed I was missing short on the studs at each butt joint with the top of the left sheet and bottom of the right. A second, harder look told me the framing is leaning to the right, obviously the way the house settled way back when, and the studs are not plumb. If I were long on money and time, I know I could rip down what I hung, rip off the existing drywall (which is in good shape, but nailed on), tear out the insulation, put sister studs all the way across (but lose r-value with the additional framing), etc...but I'm not swimming in either time or money! Is there a workable solution with the course I've already begun, like using drywall adhesive toward the butt joints where I'm missing the slanted stud & taping & mudding the joints? Or is that stuff only meant for stud-to-drywall, not drywall-to-drywall?
Regarding the upper half of the job, my naked eye shows me the bottom of the truss is nearly flush with the top plate on the right side of the wall, but runs to nearly 4" back of the top plate on the left. I assume I'll just have to painstakingly shim out the recessed truss on the left, and then add vertical studs to support the firewall the rest of the way to the roof???
Thanks in advance - lots of good advice on this site!