Key assumptions I believe you're wrong on:

neither May nor Corbyn will be able to get their parties behind it

I very seriously doubt that a compromise deal pushed by the government and the opposition leadership alike wouldn't be able to get at least a narrow majority. Labor has its rebels, but the deadline offers a compelling incentive to stick to the party line.

The tories cannot go for an election

We're talking about a country in which Boris Johnson is the most popular politician, conservatives still maintain a majority, and a majority would take a no deal Brexit over remaining. With Brexit potentially at stake, I wouldn't be surprised to see the british electorate disappoint me yet again.

and no deal is unthinkable for labour

Unthinkable to let pass, but not unthinkable to permit on a referendum -- I could definitely see labor offering the no deal option in exchange for a referendum (as has been mentioned several times).

parliament will surely choose revoke

I very seriously doubt this -- I'm puzzled as to why you would think that a parliament that won't commit to a referendum and almost refused to rule out a no deal scenario would commit to flat-out revoking Article 50. We'd see a referendum long before we'd see a straight revocation.