The fast fill feature of a PRV is not meant to be used on a closed system with just an vent open.
It is meant to be used for initial quick fill and/or purging of air. A drain hose should be connected to a drain valve on the zone being purged, constituting an open system which will get rid of the majority of air. The recommended air vents are still required in their respective locations.
Although 12 psi may me the typical pressure setting, it is not for every application. Watts recommends 3 psi at the highest point in the system. In fact their S1156F unit comes factory set at 15 psi. Typically adequate for a 3 story building.
You should determine the total height from your new pressure gauge to the highest point in your system. To convert the height in feet to psi simply divide the feet by 2.13. (1 psi = 2.31 feet of head) Once you determine the minimum psi to get you to the highest point of the system, ADD 3 psi. Chances are your required psi setting will be a bit higher than the typical 12 psi.
As previously mentioned get a gauge that is working properly. Check your expansion tank and adjust air pressure to match the new pressure setting of your system. Not 12 psi. And of course that air pressure is added when there is no pressure exerting from the water side.
I believe fixitron was hoping for additional pictures of the entire setup in the basement. Showing relative locations of all components, including air vents, air scope, zoning arrangement, etc.
Once things get squared away, individual purging of each zone, with an open drain valve on that zone, can be done.
EDIT: If you're not familiar with the proper procedures for adding air to your expansion tank or purging the air through each zone, there are numerous videos on the web covering that stuff.
It sounds like your plumber father-in-law may take different approaches. Watch more than one video on any subject as these guys do vary in their thinking.