I think it's in the bag... May just gave WTO Brexit on a platter, Europe badly needs a crisis, especially France. It is likely they refuse extension it seems with current reactions, and have stated they are prepared for WTO Brexit.
In all honesty, cancelling Brexit is the best economical option for UK, but if they choose to respect the voter's choice, between Custom Union or WTO Brexit, WTO is by far the better option... Imagine UK being reduced down to being a colony to Franco-German EU ? UK is the most influential country in the world's history, business language is English... For what it's worth, I hope for the people of UK that May/Corbyn fail at Custom Union's Brexit, they really are willing to sell out the country, the quality of life would just plummet... You lose all the benefits of being in the EU, and you can't sign any trade treaty, EU decides your trade.
Anarchy would rule UK and enter a really dangerous era, a lot of angry people would be roaming the streets, it seems best for every one if EU accepted the WTO Brexit, and it's looking like that