At slow speeds, the power generated is indeed very low, and hence there is virtually no slowing of the vehicle. Basic physics - if there is little energy being generated, there is little loss of velocity.
that's not how things work... Perhaps I still don't understand what you're trying to say.
it's not the power or energy generated/used that changes the speed of the vehicle, it's the torque or force.
in linear terms F = m*a
you apply a force F on the vehicle of mass m and that results in acceleration a. as long as force is constant acceleration is also constant.
in the rotational frame the equivalent is T = J * α where T is torque, J is inertia and α is angular acceleration. you can play around and change between linear and rotational frames using the wheel diameter and gear ratios. (F = T * r where r is the radius, assuming 90 degree angle between torque and force vectors)
Power P = F * v (force times velocity) or T * ω where T is torque and ω is rotational speed.
Power depends on speed, but acceleration does not. The torque from a motor is constant even at zero speed therefore acceleration is constant regardless of speed or power used/regenerated.
the electric powertrain is perfectly capable of slowing down the car to zero.
and the powertrain doesn't really transform anything into heat just to slow down the car. there are always losses in the inverter (switching and conduction losses) and in the motor (magnetic and copper losses). The switching losses depend on switching frequency so let's say they are constant, and so are the magnetization losses in the motor. the conduction losses in the inverter and the copper losses in the motor are directly proportional to the current going through, which is directly proportional to the torque.
these losses are always there whether the motor is accelerating or braking (and will be lower when going at constant speed but will always be there). the only difference is that when you are slowing down the mechanical power of the motor is negative (motor acts as a generator) and as long as the mechanical power generated exceeds the electrical powertrain losses, the batteries are getting charged.
but in terms of what makes the car slow down, it is the fact that the torque of the motor is in the opposite direction of rotation, and this remains true regardless of the speed.
the car doesn't start to "run the motor in reverse" at low speeds. the motor is directly coupled to the wheels through a fixed gear. when the car moves forward the motor turns in one direction, when the car moves backwards the motor turns in other direction. the electric motor in this car can't turn in the opposite direction of the car movement.
The vehicle could be brought to a complete stop (eventually), but not in a manner that allows "one pedal" driving.
EVs have one pedal driving where the vehicle slows down and stops without touching the brakes at all. Nissan Leafs do it by default, the kona does it when you hold the regen paddle etc.
the clarity could do it too if the system designers had decided so. the electric motor is perfectly capable of holding the car still even against load (as in on a hill/incline). if the car is on an incline and the motor is holding it still, then the only power used is equal to the losses in the inverter and motor.