Regenerative braking at any speed below maximum - how is it done?
The short answer is that regenerative braking is not based only on the back EMF of the motor. It's based on also using the motor's inductance (and/or an external inductor) as the key element of a boost-mode switchmode power converter that can convert the output voltage of the motor (when used as a generator) to a level that can be used to charge the battery.

Since PWM speed controllers already contain the necessary high-current switching elements connected between the battery and the motor, switching between "driving" and "braking" is really just a matter of changing the timing of the control signals to those switching elements.