If Ryland and Simon are narrative parallels/foils, so are Stratt and Ava
Both sent men off to their deaths, bearing the weight of responsibility in the face of extinction. Both had to make the hard call and take the blame, fully willing to do so if it meant saving humanity.
Where Simon and Ryland branch off in their similarities based on the state of crisis of their respective worlds, the exact same analysis can be made for Stratt and Ava. Stratt comes from a world that has not yet been hit by doom, but is fast approaching it enough that action must be taken to preserve what humanity currently has (that being a relatively peaceful, functional society). Meanwhile, Ava's world has already been ravaged by its mass extinction event. It's a world that most would not consider worth living in or attempting to preserve, and Ava's job isn't just to try and save what little there is left of humanity (that again many in the Iron Lung universe don't even consider worth saving), but to try and a find a way out from those conditions. Eva works to save a home worth fighting for, Ava tries to claw her way out of the hellhole she's in. Both are tasked with ensuring the survival of humanity.
The greatest difference, and really the only difference that matters, is that Ava lives in a dystopia and got to make the easier choice. Sentence a convict, a murderer, scum, to his doom to save humanity under conditions in which doing so is more than understandable. Stratt's world can still be salvaged, if she plays her cards right. But she has do be the first to cross the line, and she has to sacrifice a good man who she knows deserves better, one she's perhaps even come to care for. Ava didn't even know Simon's name; she didn't have the luxury of sympathy.
If Simon and Ryland are the victims of their worlds' chaos, survivors but small parts of something greater that they should be willing to sacrifice themselves to serve while being forced to do just that, then Stratt and Ava are the hands that push them towards their fates. The men desperate to live who ran from responsibility, and the women who wore responsibility like a chain, wrapping those chains around the throats of cowards and sending them to their graves to protect the needs of the many. The guilt, the desperate justifications holding just enough truth to it to keep going, and being in the unique position of knowing the weight of a crown that any sane person would avoid; that is the story of Ava and Eva Stratt.
Hell, they practically even have the same name. It's just a letter off.