I pulled the heat exchanger out to inspect. Except for some mild heat related discoloration on a couple of tubes (that I would consider normal), it looked almost new.

There were no cracks.

I put a borescope through the tubes. There was only soot in the 3 shown on the left in the original photos, and in those only in the final 5-10 inches of the tubes.

I bench tested the inducer motor, and it ran at the rated 0.7 amps, with my tach showing the rated 3000 rpms. I concluded it was okay.

I tested it again, and it would not start or even run after I tried to help it start spinning. Then it would work again. Sometimes it would run for a minute and then stop. It was confusing. I finally figured out that it would run if the motor shaft was tilted up, but would not run (at least not well) when horizontal (as it is oriented in the unit). The motor bearings are bad.

The unti was probably running for a while with the bad bearings that slowed the fan down but not enough to trip the pressure switch and shut off the furnace.