How is prompting Dave to remember multiple timelines meant leading to saving Oswin and Roswell?
Was the idea that Dave remembering would change the family's morale and convince them to try the cure?
Because after all, they can't know if the cure works until they try?
Well, for Roswell it's easy. Dave remembering the vacation means he won't be able to follow with his plans. So it forces him to reevaluate things. Oswin is the one that i just can't find the reasoning. My best guess is seeing Roswell distraught might make him more willing to present the option of the medicine. It's a stretch but i can't come up with something better.