The power is limited to the current in the Sevcon.
When the battery voltage sinks, the current will go up.
There a several limits in the Sevcon.
Some of them are linked to the temperature of the controller itself but also for the temps of the motor and battery.
There are also timers in the Sevcon that limit the current levels to 10 and 30 seconds and permanent power.
Driving a tuned twizy for 45-55km on the motorway flat 100km/h with ca 18kW at 20 C will not result in overheating.
The motor will set at 130 Centigrade and the speed will go down with the voltage.
You get more problems in the city with high recu and frequent sprints without air to cool the motor/controller.
Got the motor up to 160C and the battery up to 60 C.
At taht level the Sevcon will reduce power and recu.
When the motor reaches 180C the sevcon will cut power significant!
This ist the max rated temp for the motor.
Take a look at :
A lot of your questions should be answered in that doc.