So Parliament is going to sit again. No doubt it could, with Bercow’s connivance, pass all sorts of laws. But in the end, it’s the PM who has to ask for the extension. And Boris can still refuse, whether outright, or by way of some legal cleverness. So the Remainers in Parliament still really have only two options.
One option is to outright revoke. I wouldn’t put that past them now, but they’ll be signing their own death warrant if they do. That will result in widespread outrage, and intense pressure to have a new election, which will have to happen sooner or later, and that will result in pro-Brexit party or parties winning, and they’ll just pull us out of the EU straight away.
However, the wild card is how many Conservatives vote for revocation. A handful the party can cope with, as long as they are immediately expelled and replaced. But suppose fifty did? It’s hard to see how the party can get over that. That would make it pretty clear that the Conservatives are riddled with anti-democrats EU fanatics. There would be a mass defection of voters to the Brexit Party, but the worry would be that not enough defect, and both parties scupper each other.
The other option for the Remainers in Parliament is to finally have a confidence vote in Johnson, and then vote in a new Remain PM, who will go to the EU and get an extension. But the electorate would also be outraged by Parliament appointing a Remain PM who crawls off to the EU, and an early election would have to take place sooner or later, which would again probably be won by pro-Brexit party.
Another serious risk for the Remainers with this strategy is that it may be that no Remain figure gets enough backing, and a general election has to take place instead, which the Conservatives would probably win, with most of the Remainers losing their seats.
The wild card with option two is the same sort as with option one: how many Tory MPs would vote for a Remain PM? If you got fifty or more doing so then the previous intra-party schisms will look like a disagreement at a debating society, and the Conservatives would struggle to keep themselves above water, and they may take down the Brexit Party with them.