I think Labour should have stood down in marginals where they're out of the race, not only does it help potential tory-ousters, it would have put pressure on lib dems to stand down in the real crunch lab/con marginals where the libs serve only to shit things up.
Two reasons why they didn't (IMO): 1), I imagine the tories and MSM would have pasted them for doing so, they would have been accused of abandoning places, not being a truly national party, etc. The tories have already been complaining about the idea of tactical voting this week, so I can't imagine the screeching if there'd been an actual agreement. You know, like the Brexit party stepping aside for the tories. 2) Neither Corbyn nor Swinson wanted to make the first move. They didn't trust each other to reciprocate, whether officially or unofficially.
Lib Dems standing aside would have meant admitting that all the misleading bar chart leaflets saying how well they did "last time" they sent out were misleading.
Current Lib dem economic policy of austerity forever would be more suited to a tory coalition, though Corbyn is obviously far more suited to liberal values generally, regardless of how much Ian Dunt weeps about him.