In my reading, Drake is saying, if he were in Mysa's position and he had an option, however horrible, that would let him be Dawny's lover, he'd do it; and she's saying she'd take Blok's part and be his partner in that choice. Trouble is, the universe (and Paul Levitz) seems determined not to present them with one.

Yeah, see, glass half-empty, glass half-full. I'm reading the exchange in pretty much the same way, but coming to a different conclusion.

The 'could', specifically, leads me to the opposing interpretation - the choice that Blok had to make was not an ideal one, but it's the only one he could make to be with Mysa. So, for the parallel, if there was anything Drake could sacrifice that would allow him to be with Dawny, he'd sacrifice it, and she knows it. But, there isn't, so they're unable to be together.

If they were together, I would expect the exchange to be 'if I had to', not 'if I could'.