the plot thickens:
If I jump the relays, the fan spins at high speed, but not at low. That makes it pretty clear that the fan motor is fine.
Jerry from Sweet Motors confirmed that my fan would be 330 watts, since it is a two speed fan.
What he found odd is that the resistor wattage value should be between 50 and 75 watts, as reported above, with a 300 watt load.
I am not an electrician, so if anyone can shed light on this, I would love to understand it.