Got a replacement blower #ECM#motor from northamericahvac.com - had to order from the States. Installed it, and ... furnace was still throwing an #error code meaning "incorrect #signal from ECM motor" once, then "no signal from ECM motor" forever after.
After double-checking, I saw the part number on the box didn't match what I'd ordered - but that turned out to be a red herring. The box number was for the unprogrammed motor.
Did some digging and discovered the #coil in one of the #relays mounted on the furnace control board had apparently melted. I haven't mapped out the board to confirm this, but I suspect it was the relay that powers the draft #inducer motor. Without that spinning up, the furnace doesn't get the #exhaust draft [1], and so the pressure switch doesn't trigger.
The furnace refuses to light, rather than #kill you with exhaust gas leakage.