IIRC, the CI spell was towards the sides and upstream, what means that killing Susan won't kill Maggie by the spell itself. Then, if you consider the vampires themselves, Susan was the newest vampire so, worst case scenario (Maggie being infected, like Susan before her complete transformation), the vampire part would have done at the moment, leaving only a human girl. That was what happened with the other infected people.
Still, I admit that the two things you highlight seem to be hinting something. I just don't know what.