"ad 1) irrelevant, you can make the first instruction start the WD."

Correction:

1. it must be enabled by any presence of power.

Maybe a short glitch cause restarting the CPU but was to short to trigger a full power on reset. Then the CPU starts from anywhere and the WD enable was never executed.


"ad 3) if it does, it will wake up the processer at timeout intervals which can be desirable in some cases but in other cases it will be unwanted."

On some modern micro controllers you can program a fuse, that the watchdog was permanently on or not.
So if lowest power consumption wins against reliability, then let the watchdog fuse unprogrammed and enable/disable the watchdog by software.


Peter