I don't understand your question, but from what I can gleam...
For a given load, the duty cycle of the PWM would be varied to control the speed. The problem is if the motor gets more heavily loaded and slows down, more current is being drawn which would overheat the FETs. As the motor slows down, the BEMF sampling frequency (which is the PWM frequency) can be lower and still capture the commutations properly. I want to decrease the PWM frequency as the motor gets more loaded to decrease the switching losses on the FETs. So as the motor slows down in response to whatever load happens to be on it, I want to slow down the PWM frequency as well (independently of the duty cycle) to reduce switching losses as the I2R losses increase due to increased motor current.
But, the PWM frequency is not being varied to control the speed...it's more like it is being varied in reaction to the speed rather than to control it. I'm not attempting to control the speed of the motor by changing the PWM frequency- that's what duty cycle is for. But I do want to reduce the losses in the FETs. I can't do much about the I2R losses resulting from motor current, but I might be able to do something about switching losses.