The issue of the studs not being plumb (laterally within the wall) shouldn't be an issue. From the pictures, they don't look off enough to be of structural hazard.
As far as them not being "flat", how far off are they? You say a 1/2". Is that the worst case?
I would recommend putting 1x strapping perpendicular to the existing framing if you are really concerned. that way you can shim and fasten them easily and it will give you slightly thicker walls that you can put either homosote of rigid insulation in.
http://cabothousecondos.com/index-additionalfeatures.html This has a picture of strapping that is perpendicular to the framing. This website pertains to sound proofing, but the concept is very similar.
As far as them not being "flat", how far off are they? You say a 1/2". Is that the worst case?
I would recommend putting 1x strapping perpendicular to the existing framing if you are really concerned. that way you can shim and fasten them easily and it will give you slightly thicker walls that you can put either homosote of rigid insulation in.
http://cabothousecondos.com/index-additionalfeatures.html This has a picture of strapping that is perpendicular to the framing. This website pertains to sound proofing, but the concept is very similar.