The Benn law where it was rushed through Parliament is full of holes.
If Boris tries:
1. To get a deal & it is rejected by the EU he can probably get away without asking for an extension.
2. If May's BRINO is re-voted on and rejected he can definitely not ask for an extension.
3. If May's BRINO is re-voted on and passed, PM prerogative is not to complete the ratification and again he needn't apply for an extension.
4. Whether Boris can be forced to seek an extension with contempt proceedings is open to question under the Bill of Rights and also the 1947 Act that specifically excludes crown v crown legal action.
5. All this has got to be decided , appealed & judged by the Supreme in under 2 weeks or we leave.
6. Finally the nuclear option is for the crown to concede the Robin Tillbrook legal action that the extensions were illegally passed by PM May, so we actually left on the 29th of March.
The reason that Remainers are currently having a meltdown is that by the 1st November, it is odds-on we will be out of the EU.
Expect the last 2 weeks of October to be a 'living in interesting times' event, the likes of which, we are unlikely to ever see anything like it again in our lifetimes.