But why would the Reds kidnap her then in Grave Peril? If she was already nemfected before, she would have been in the correct place already. They kidnapped her an Susan, but did apparently nothing to Justine while on the other hand they turned Susan. Why kidnap Justine in the first place, if not to put Nemesis inside? So Nemesis was inside Justine and held her stable until Blood Rites, when Thomas nearly killed her. I'm convinced, Thomas would not have been able to stop himself, true love or not. He didn't even remember, he stopped feeding. I think, Nemesis pushed him back, to keep Justine alive. And I have the suspicion, Nemesis and the Outsiders created this situation, so that N. would have the opportunity to leave Justine by herself for a while (explanation: almost death by White Court feeding), so he could concentrate into playing out his scheme with Lea in that time and setting up his plans with Maeve.
I think, Justine was and is the main host, planted in 'the middle', able to get in contact with most players, just an innocent and harmless human 'doe' with questionable sanity. People would talk carelessly in her presence and she could transport Nemfection to every being that needed to be controlled, without anyone thinking twice about her weird behaviour.
Further I think Lea was just a distraction for Mab. She should think, she succeeded in exorcising the Walker, and be occupied as long as needed with the deed, so he had time to prepare his real plans.