What happens if the BEMF generated while going downhill is higher than the supposed applied voltage to the motor terminal by the inverter?
The motor will brake, at least initially. The braking energy will be returned to the inverter. If the energy is not dissipated in braking resistors or returned to the battery, it will be stored in the DC bus capacitors. That will increase the capacitor voltage. When the capacitors fail, the energy will be absorbed by their remains until they become an open circuit. At that point, the inverter will probably have failed in some manner. When the situation reaches the point that the motor is attempting to push energy int an open circuit, the motor will stop generating and coast.