Not well after, at the end of Blood Rites Justine is in a wheel chair having almost died saving Thomas. She becomes Lara's secretary shortly after.. Problem, Harry calls it Nemeses, an Outsider, and a Walker.. We know that a Walker is an Outsider knight, but are Outsiders really Nemeses?
I don't follow the last question.
The Outsider Walker named "He Who Walks Beside" is commonly known as Nemesis. He's a specific Outsider, who specializes in infiltration. I rather think Demonreach was setup to deny him access to hosts he'd otherwise infected, and therefore free to deny their natures.
On the overall topic: Nemesis-infected Maeve told Justine she'd be seeing things Maeve's way soon. This is either recognition of a latent infection, or identification of where Nemesis "jumped," if that is its mechanism, when Maeve died. At that point, a couple of years prior, Justine was an ideal target, close to Lara, etc. I rather think people are looking too far back for Justine's exposure.