Roswell isn't in control anymore. Dave remembers, so everything Roswell had been tunnel visioned over, everything that he'd 'decided' was now incorrect.
This is why I feel that there's no good way of explaining this without having a mini-lecture on quantum mechanics in-game. Roswell observed reality as him being doomed to die, so it was true. Oswin observed his medicine was a failure and that saving his son was out of reach, so it was true. Florencia as an agent of chaos decided to test this and thus didn't observe anything beyond seeing her family needing saving from themselves but within herself was incapable. Dave recalling the events of the game observed an option and existed as a contradition to Roswell's observation. If Roswell was observing reality one way but concrete proof in front of him directly contradicted it, his reality flips to the inverse: He's not doomed to die. In the same way, Oswin's observation in light of this is the same, flipping to 'his son can be saved, his medicine wasn't a failure.
Florencia, who understands quantum mechanics, decides to open the door witnessing the way the universe is now being observed and thus opens the vault in the same way we open the box and see if the cat is alive or dead.