Justine reacted rationally first, then broke down. It's a little weird (but I don't think totally abnormal in reality), and it pings in Harry's mind as smart and that he had underestimated her. Then he leaves to do his investigation around her building. At that point, Justine has some breathing room; she's rid of Harry for a few hours. If Justine does nothing (and Eb hadn't shown up), Harry doesn't find anything and goes to talk to Lara instead of getting patched up by Butters then talking to Lara. Nothing significant changes in the plot if Nemesis doesn't make a move there compared to summoning cornerhounds. That's my point in that it doesn't fit Nem!Justine's motivations well.
What are the motivations of Nemesis then in your estimation? Back to your point of Justine being rid of Harry, physically maybe, but that in no way makes her rid of him. No, I think Harry's gut reaction is dead on to Justine's reaction, "does Lara know?" Sorry that isn't normal, especially when you add what Eb says about a star born like Harry and why he poses a danger to Outsiders, he can see through their crap! That is why the boat trip ultimately fails for Nemesis, when it is calm, even though exhausted, Harry finally puts it together, he had seen though her in the apartment. Justine asks if Lara knows, not because she will help Thomas, but because she will be sympathetic to her! Lara won't be able to see through Justine, but Harry has that ability, she needs Lara to take her side. Which she does and insists that Harry take her to Thomas on the island, on the boat.
So if you think Nemesis needs cooperation and motive for a mortal to summon outsiders, and you think Justine summoned the cornerhounds, you're saying that Justine is willingly cooperating with Nemesis against Harry? I don't buy that at all.
You are totally ignoring the fact that we are now talking about infected Justine... Infected Justine who admits on the boat that she worked to get pregnant to set Thomas up. In short, she is no longer playing with a full deck, an insane person still has free will, it might not be rational, but as a mortal she is free to call up the Cornerhounds. Think Justine of the boat trip at the end of Battle Ground, not sweet Justine of the earlier books. At the beginning of the trip she was so sweet so concerned about Thomas, she hugs Harry.. Then she asks why Thomas did what he did? Then she questions Harry and sort of confesses "Were they trying to use
us,against him?" Us? As in her and the baby? Who was? Then Harry goes to rest because he is exhausted. He starts to replay all of what had happened in his mind. 1] That Thomas had been trying to say Justine. Not calling out the name of his love, but that she made him do it. 2] Then he returns to the scene in the apartment,
"Something about Justine wasn't. . .quite right, earlier, in the apartment," And I let my voice harden. "How long ago did you possess the girl?"
The sweet Justine kind of melts away, just that quick.. Oh the body is there and the beautiful face, but it isn't her at all. Then He Who Walks Beside, explains who he is and his mission, which should answer all your questions as to why Outsiders aren't summoned up as armies to get it over with it just isn't their style... They'd rather go about it this way;
The she exhaled in a slow, utterly sensual voice, "I am the doubt that wards away sleep. I am the flaw that corrupts, the infected wound, the false fork in the trail. I am the gnawer, the worm in the book, the maggot that burrows in the mind's eye."
Justine exploited all of Thomas's weaknesses, his love for her that was Justine. Corrupted him so he'd be driven to murder and hopefully shatter the Accords. Sent the Cornerhounds to throw Harry off the scent and blinds Lara to what she really is.
One more answer to your question, Harry asks her, "why? to shatter the Accords?"
Nemesis answers;
"Apocalypse isn't an event," Nemesis murmured. "It is a frame of mind."
Then it goes on to speak of an unraveling of all things ending in Empty Night.. Interesting
it happens slowly and is done before anyone notices.. Madeline Albright in her book about
how democracies die describes it this way, paraphrasing, " the forces against democracy pluck away
at democracy feather by feather, like one would pluck a chicken, no one notices it or feels it until suddenly all the feathers are gone.." And people find they are living under a totalitarian state.
Nemesis basically tells Harry the same thing, how they will undermine everything, Mab, the Accords, upset the order of the universe, then all they "need is a single opening."