Have you tried putting the /stopcasting *after* the Exorcism?

Code:
/cast exorcism
/stopcasting
I think your problem is that starting to cast Exorcism when Art of War hasn't procced will trigger the GCD. On your next keypress (before GCD resets) all abilities that require the GCD to be up will fail. I don't think you can macro it so that it doesn't trigger the GCD if Art of War hasn't procced.