Thanks for all your thoughts, Greg. I should have mentioned that the scorching the earth stuff was just figurative, just a way for me to express how it felt they were treating each other and teaching a lesson within the relationship.
Also, I've got this part of the story written already (first draft at least) and so I know what's happening on the surface anyway.
Act 2 has them learning various things about their relationship, and the process of learning is kind of tough, but it makes their relationship grow ... until Becca's had enough of Devin's misguided help and rejects him utterly*, which is like teaching the relationship a lesson, that they need to stop considering that they're close. That's the earth-scorching, which happens near the end of Act 2, so far as I can tell. Then RS Signpost 3 is Understanding, so I think that lesson moves into them understanding that they can't rely on each other, that they're going to have to be careful trusting each other. (I'm right around the midpoint in my first draft.)
* some of that rejection is in the IC and OS throughlines too. The OS is mostly where his inept Help was. (Some of his Help was good too, but even that she found annoying. That's the OS Problem for you! )
Of course, I'm aiming for a big Understanding moment later on in Act 3, which is when Devin has to talk Becca out of committing suicide by jumping off a rain-swept bridge. That's where they'll begin to understand that they really do share a special bond, that they need each other.