If I frame a new ceiling without removing the old drywall, then add insulation, do I get the r-value of both new and existing insulation? The space between the old and new ceiling, is this going to trap moisture - the inaccessible attic will be passive vented? Thank you in advance.
You would add the r-value there. How much paint on the drywall?
If there wasn't a sign of moisture accumulation before, you should be fine because it was vented. There is the potential that the old ceiling plane will now be much colder with the new layer of insulation and drywall.