Wrong. Lots of people came up with ideas that involved that. This is not because those options were better. heck, it was not even because they were any good. Generously, it is because those ideas were more dramatic - fit a certain kind of story logic. They were also very, very likely to fail, because they were much too complex. Far to many things would have to go just right in order for any of them to come off, most of them not under the direct control of the plotter. The only timey-wimy gambit I would even attempt in their place is the one I suggested - a substitution of the injection Harry gave her, because that is a single change that does not violate the observed course of events.