Think about it, Ace was captured at Alabasta and wasn't set for execution until after Water 7/Ennies Lobby. That's a good bit of time. So say they missed Robin she goes to Imple Down, SH's learn where they take here in the process Luffy hears they also have Ace. Luffy breaks into Imple Down as he would have anyway and now saves Robin, Franky and Ace simultaneously. Either way Robin would live and Luffy would go to Imple Down.