Need to know how many people Nemisis / HE WHO WALKS BESIDE can infect at a time.
Did it jump from Leah to Justine? Were Cat Sith, and Meave possessed at the same time? If so, Justine as well?
How far can it reach, and can they possess a Starborn?
We have every reason to believe that Nemesis can infect numerous beings at once. We know that Aurora was infected at the time of SK events. Leah was infected somewhere between GP and PG, but the line in SK where Maeve said she just came from Leah, without any other plot purpose, points to her being exposed at that time.
Later during PG, Leah is infected at the same time Maeve is. Then we have the potential that Elder Fetch was also infected in PG, so possibly another case of up to three at once.
We know Maeve and Sith were both infected in Cold Days, so there's two again. I don't think Justine was infected yet at that point, because Maeve specifically called out that Justine was close to Lara and that she wanted to talk to her. Everyone should have known Justine was a target at that point, but I think the lack of understanding of Beside prevented them.
Then there were a whole bunch of people listed by Lily in Cold Days that were infected and/or influenced by Beside, which I think at this point has to be considered accurate. That included Sells, Denton, and others. So at least three, if not more, at once. I'd like to think there was a cap of three to puppet at once.
All that being said, I still hypothesize that Black Magic (killing with magic, along with some or all of the other laws of magic) opens a door in the psyche/brain/spirit/soul, and it allows the corruption to seep in. The corruption isn't Nemesis/Beside, but he can use the same door to enter and manipulate people.
That's why the Council takes such a hard line against the law-breakers.
In Justine's case, the question is who infected her, how, and when. It didn't seem to happen on the island during Cold Days, but it could have been done during her kidnapping prior to that. But that would mean someone else on the Red Cap's team infected her, and Maeve was just catching up with her to-do list on the island.
If not during the kidnapping, then it was likely sometime between the end of Cold Days and Peace Talks. It was likely in the events prior to Even Hand, where she's just escaped from Corb's people with the prisoners and the girl. Maybe it didn't go the way she remembered, but Beside manipulated her memories.
That would make the story more relevant to the overall plot, as it illuminated the specific event where she was compromised.