Outside of Thomas taking up Amoracchius and using it to somehow remove Nemesis from Justine, what other method might be available?
I was thinking about a piece of the Wall to be used as a foci/ritual object. Presumably it's an anathema to them, or they'd storm the Wall rather than the Gate. Maybe it could help get Nemesis out? And possibly help keep him out after-the-fact? Would shaving off a little bit of the inner Wall be worth making charms for Harry's closest allies to keep them free from Nemfection?
In some sense, she must be able to, because free will.
Thomas appeals to her, Justine, his love for her, her love for him, their love for their baby. She then kicks Nemesis out of her head. Yeah, I know it will be more complicated than that, but if her will and love are strong enough, I think she can do it.