" Blower comes on, cycles through, and releases gas but the igniter never glows"
This observation indicates the pressure switch is working fine. If it were not you would never have gas flowing.
"Disconnecting the connector to the pressure switch and checking the voltage at the connector it remains at a steady 24VAC"
Does the above happen with the gas flowing too?
I would double check your AC voltage measurements. If you look on the control board you should find a small relay that's used to power the HSI. Possibly that relay is the problem. Check the board for any signs of loose connections/cold solder joints. You can always jumper the pressure switch connector to totally remove the switch from the circuit but I doubt it's the switch... they work or they don't.
Another possibility is the gas valve solenoid, when activated opening the gas valve, is causing the voltage drop. You can measure the impedance of the solenoid coil and see if it's correct. Also check the transformer connections to the control board.