I can see Devin being the one to interrogate Zack, and it would bring out the side of him you like so much,
Yes please. :)

I think part of the problem is the way Calla just blurts it out while sitting there. Or maybe it's because she's a robot so there isn't a lot of passion in it (despite her being mad). Maybe she just needed to get up and be intimidating with flashing eyes. Which is how I picture Devin going about it (minus the literal flashing eyes or course).

I'm thinking that despite what I mentioned in a comment below (about Devin being drawn to Calla because she hates him), it might take a little bit more for him to open up. Like, he might show up and then just sit there and not say stuff. To the point where Calla would say something snarky, wanting to know why he's bothering her, and eventually he'd end up talking more.
That would work so much better. You could have him snark back a bit, snap at her. Then next time he opens up slightly (unintentionally) building up to him being more comfortable.

If you do you could also switch the POV for one of these so you could see Devin struggling with the urge to blurt things out to Calla and he doesn't understand why he's drawn to her since she hates him. Maybe he even gets an intense urge to find out why she hates him since he's never done anything to her. Just to quell his curiosity. See, now I get more fanfic plots in my head. If I give them all away what will I write?

Interesting about the families. I'm intrigued.