Controlling the "amount" of regenerative braking (variable back torque?)
The amount of torque a motor generates is dependent on the current flowing through it. Control the current, and you control the torque.

You can control the current when accelerating using your standard acceleration controller. 

When regenerative braking, the controller that charges the battery from the motor generated power is programmed to draw a small or large current from the motors, generating a small or large braking torque.

The maximum torque a motor can generate depends on the maximum current you are prepared to put through it. There are several limits on motor current. Amongst the limits that directly limit the current are we must not demagnetise the motor, tear the windings out or break the shaft. An indirect limit is the maximum temperature of the motor. It will have a continuous rated current, which it can run at all day without overheating, losing heat continuously through its ventilation. This current will be an order of magnitude lower than the other direct limits, so it can be run at more than that for a limited time. For example, it might be able to run at continuous+50% for a few minutes, +100% for one minute, absorbing the excess heat by raising the motor temperature.

I don't know whether commercially available systems make use of this, but if I was an electric automotive designer, I certainly would. When motoring, especially if going up a hill, there is no limit to how long that would be required, so you must limit at the motor's continuous rated current. When braking, we know the maximum amount of energy we are going to have to handle, so can afford to operate the motor in a time-limited overload, and stop with a higher power than we'd use to motor.

How 'taking your foot off the gas' is programmed to control the vehicle is simply a software/usability issue, it will work however it's been programmed to work. As a driver, I'd prefer foot off the gas to equate to no applied power, no applied braking, which some folks call coasting. Pressure on the brake pedal should control the braking torque, so that it drives like all other vehicles drive. Obviously there would be significant safety and UX engineering to happen into exactly how the brake pedal was mechanically linked to the friction brakes, and controlled the regen brakes, but that doesn't change the physics of the motor and its current control.