Blizzard, through their API, has determined what is ok for an addon to do and what is not.
If key state actions (up or down) are fine for addons (see Keyboard API), I don't have a problem using them.

That said, if you do have a problem...
Keep the mapped key, as two steps.
But set is to execute only on key release.
Press (and release) once to initiate the IWT.
Press (and release) a second time, to end the movement.
The speed between first and second press can be as quick as you like, there are no GCD restrictions.