You can run the motor to the maximum duty cycle but you would draw a lot of current and your FETs will overheat that is why employ heatsinks. For the low duty cycle, this will depend on your load. I can run the BLDC to 8% duty cycle but below 8%, the motor does stutters due to loading.
If you increase the PWM frequency, your mosfets will heat up more, but your AC ripple current will be smaller. On the other hand, as your ripple increases you will get more core losses in your motor. The maximum frequency will be dictated by your mosfets' cooling and the PWM resolution you want (if digital control).