There usually is no pre purge. A call for heat starts the powerventer. The pressure switch makes after proving the fan is still working and now sends power back down to your primary control. Which then starts your burner which may have a pre purge setting built into your primary control.
I have changed out those pot switches a few times without issue. Always solves the problem. Because you have no natural draft you need the fan to run to remove the residual combustion gases after the burner shuts off. As you know they now vent out into your house.
You need to find someone to...
To find common on your transformer, a multimeter would help. Set it to ac volts, one lead to a terminal on the transformer and one lead to ground(metal). The terminal that gives you a 24 volt reading is your hot. The the terminal that gives you no voltage reading is your common.