AFAIK, you can (sort of) think of a DC motor as a constant current device.
1. The resistive component will tend to decrease the current in the motor if you decrease the voltage (ohms law).
2. The decreased voltage also leads to slower turning speed and less counter electromotive force (thus bringing the current back up).
Nonetheless, the power consumption goes down (slower = less work being done).
I've never heard of a fuel pump quitting due to an alternator deactivation myself, but that isn't saying much. I know I've driven home without an alternator belt on a few occasions. EOC with bump starts and a few cooling off breaks plus pulling the RAD fan relay was the only thing that got me all the way home. No complaints from the rest of the systems, knock on wood.