Hi Tarik,
Yes, the SIM_COPC register is a write-once register after reset.
The COP watchdog is still enabled in WAIT or VLPW modes because the bus clock and other clock sources are still enabled in those modes. If you want the COP watchdog enabled, and you would like to use those WAIT modes, then you will need to wake the core periodically to service the COP watchdog, like using another hardware timer interrupt.
Thanks