How can install rafter baffle venting without removing drywall?
Typically this type of structure is properly vented already as you described it. The pitched part of the roof is typically insulated tight with a 1" sheet of foam at the roof then friction fit unfaced. Then a plastic vapor barrier then drywall. Sounds like you're doing a roof so you might consider a foam roof substrate over the pitched rafters. Your existing  Kraft face insulation could be causing tagger show because of leaks in the seams. Consider a vapor barrier paint for the interior.