I have a Sea Era saltwater model. The only failure I've had was the push button and Raritan replaced with with an improved model.
Is there any chance that your batteries were low or some kind of object had jammed the impeller? The flush end of the Sea Era looks a lot like a standard macerator, and the unit pulls a good 15+ amps when it runs. That is a lot of power that wasn't able to turn the motor. But since you shorted the push button switch and it still didn't run, that would seem to eliminate the switch as the problem. BTW, my switch that failed had a plastic body and the replacement was a metal body.
So it does sound like the motor was stuck or seized. Was it difficult to turn the chopping blade?
I carry a spare rebuild kit for the discharge end and the intake end. The only thing I have ever replaced was the joker valve after 5 years. I don't carry a spare motor. If I didn't have a second head I might consider sparing the motor.
FWIW, if you have a second head as a backup, I would wait to see if this happens again before buying a new motor. A quick search showed it to be about $120-125 street price at Defender plus shipping and tax.