One thing has occurred to me - the 19th wasn't a good choice of deadline for the Benn Act, because it's a Saturday. That means that if Johnson defies it, MPs aren't actually going to do anything about it until the 21st, which gives them 10 days to sort out a vote of no confidence, forming a new government, and getting an extension. The last shouldn't be a problem, since the EU are currently working on sorting all that out from their end, but the first takes time and will undoubtedly see Johnson trying his best to obstruct it, and the second looks like it's going to be an epic ***********.