You're absolutely right, of course—having Keith experience the consequences of his choice would completely alter the shape of the plot, and the shape it's already in is a very elegant and fitting one and perfectly structured around the prophecy and all its narrative fatefulness and finality. Yes, and that's a good point about her other use of the same tool as well.

(...still, couldn't he have chosen to warn Ewen, done it definitely, and then got stabbed? That would give us part of it...)