I have my suspicions that this engine has a 12:1 CR and needs 50* at idle. The 15" of vacuum & needing that much idle timing is also odd. At what rpm was the vac measured & was the trans in gear if auto?
If the mixture screws are backed out a long way, but still adjust the mixture ok, it suggests the engine is getting extra air at idle [ vac leak?? ] from somewhere. One place can be the PCV, but with 15" of vac, I would expect the pintle to pull in & close off the air supply [ which would be normal operation ]. With poor vacuum, the vac is insufficient to overcome the pintle spring & a LOT of extra air comes through the PCV port on the carb, into the engine. If you put your hand on the PCV, engine idling, & you can feel it 'rattling', it is not working.
As I said, some of the info quoted seems contradictory.