The draft inducer blower and its air flow check switch may have been necessary when the house way built because of the chimney design? I have built 2 brick wood burning fireplaces in my lifetime and both worked great because I followed design guidelines in books I read. A well designed chimney will, even when cold, "pull" a draft. (well, maybe not all the time). A gas fired boiler or air heater lets gas and air into the combustion chamber and if the gas does not ignite when it should, could go BOOM if the accumulation of gas is not scavenged, if thats the correct word by the draft inducer fan. the fan has its air flow switch that makes sure that there is sufficient air movement through the flue pipe. Oil burners have their own blower, as you now know. Now you got me interested in your setup, so I need to learn more about it! My Priority zone relay box is one I designed and built by myself when I installed my Super Stor water heater tank. I needed a box with a total of 5 zones, one of which is the priority zone for the heater tank. What was available at the time I looked had only 3 zones plus one priority zone if I remember. it was not a big deal for me because I am involved with industrial machine controls, and I knew what the box had to do.
So now I have another question that maybe you already answered :-) during the period before lock out, does the draft inducer fan or whatever you got is called run as far as you can tell?