I think we're just in disagreement here.
I'm not suggesting some sort of joint Corbyn/ Liberal Tory campaign, I'm talking about the idea of an opposition putting such pressure on the government that the media and sympathetic MPs turn on them, forcing a policy U-turn. That's a pretty common event (see NI rise, tax credits cut, grammar schools).
This is the time to attack the prospect of a hard Brexit, while momentum is against May and her vision. But Corbyn's team aren't interested, and while the opposition support the same colour of Brexit as May, the growing calls for a change of tact (see Osborne, Davidson, Umanna, Clegg post election) can be ignored much more easily.
It would also be a huge political win for Labour if they managed to shift the direction of Brexit. But again, you have to assume this isn't something Corbyn and McDonnell are interested in. They've long been sceptical of the EU, have at-best lukewarm opinions on capitalism and haven't focused at all on the benefits of the single market.
I was shocked by how much of the remain vote Labour appeared to hive up in this election. Sadly it doesn't look like they're going to do anything with it.