What would happen if your guards didn't manage to imprison Aquitaine?
I would have cried. And then, I would have put gin in my Corn Flakes and narrated a much shorter film about how I threw away 1/3 of France in a risky venture.
The only good news is that since France's crown authority is already really low, letting Aquitaine become independent in a civil war would not be super bad, just plain ol' bad. If crown authority was higher than minimum, letting him go would mean a reduction in crown authority. That really sucks if you need crown authority.
Yes, in case it isn't obvious, what I did in my game was a risky strategy.
Although, your post does remind me, Delra, that with changes in 0.80, there might be other options. I could let Aquitaine get away with a plot to make France an elective monarchy. I could then confiscate William's lands and make my brother Duke of Normandy, and vote for him as the next king. He would vote for himself, and I could start handing out duchy titles to stack the deck in my favor.