FIrst, MA is very strict about homeowners doing plumbing work...I think you'll need a plumber.
Second, many places won't let you insert pvc between CI sections...the weight can crush the stuff. You may have to remove the CI above to the point where you want to replace. The easiest way to connect CI to PVC is with a no-hub connector - this is a reinforced neoprene sleeve with a metal jacket and tightens with hose clamps.
Third, putting a jog in the stack can be done, but will take up more room than you want since you shouldn't use 90's to make the jog. There's a limit on how much change of direction you can have (not sure of the limit).