So I think that Eb's reaction to them is probably important.  He also seemed to know a lot about them hinting he may have dealt with them before.

It could mean that, or it could mean that in his three hundred years of life as a very powerful wizard, Eb would know a lot about them and may have fought them before. 

Butters also could be wrong about who they are, yeah he is a Holy Knight now, but has he been given a full course on the supernatural?  Michael hasn't. 

If it was Harry messing with time, it had to be another Harry, because the one telling the story doesn't seem to be.  If it was Eb, why would he risk that kind of attack upon himself and Harry by time travel?

I think we are making this more complex than need be, go back and read what HWWB tells Harry on the boat just before they get to the island. 

Mab told Harry that she and Uriel had a common enemy when Arctis Tor was attacked, and Harry believed it was Thorned Namshiel.  However Harry, who is pretty darn good with fire didn't think he could have done that kind of damage even with Hellfire, in the heart if Winter.  Has it actually been confirmed that Thorned Namshiel was the culprit?

Maybe he couldn't but then Harry isn't a fallen angel, there might be some things he just has no knowledge of.  Harry also constantly talks about older wizards who just through sheer experience and study are way more powerful than he is, well Thorny has thousands of years if not more on him.

And Harry missed the opportunity to ask that question. He probably would not get a clear answer but he was entirely justified to ask at that moment.

Yup, but he also had more pressing problems at the moment.

Sure, it's a little weird, and with time to think it probably helps Harry put the pieces together on the boat.

The point is, he knew it was off when she said it the first time out.. Just like in his conversation with Cat Sith, he pressed and Nemesis revealed itself.  He didn't press Justine in the apartment because he was already distracted by what Thomas had did,what had happened to him and his fears for Justine.

But taken over, Justine wasn't the one in control.  No mortal free will there to summon outsiders.  I'm saying she wasn't just Justine at that point, so she couldn't summon outsiders.  (Thomas got it on with Nem!Justine.  It was HWWB in the driver's seat.)

 
Do you not think that it is possible that at that point Nemesis could threaten, Justine, Thomas, and more importantly her baby?  The threat was enough for Thomas to foolishly try to murder and disrupt the Accords.. So is it not logical that that kind of threat would push the mortal Justine to call up the Cornerhounds? Yeah, it was HWWB in the driver's seat, and he pushed Justine to blow the horn.   

Nah, they aren't sneaky because they want to toy with reality.  They're sneaky because that's their only option to destroy reality.

Thank you for making my argument... You are answering your own question as to why they just don't get someone to call up a whole army. 

Raith tricked Madge into helping him.  It's not the same thing as possessing her.

No it is not, but the point is, he got a mortal to help him call up an Outsider..  Justine wasn't always the sane little thing we see today let us not forget.. Nemesis occupies her brain, but her mind still exists, no doubt they communicate, who knows what Nemesis is telling her, it could very well have tricked her into seeing Harry as come kind of threat to her and her baby, thus the mortal Justine does what needs to be done with a little umph from Nemesis to call up the Cornerhounds.

Yep, exactly.  Nemesis can't force mortals to summon outsiders, or the Outer Gates would be barely a speed bump for them.  At best, it can trick them into doing it willingly.

Which it did, I don't think Nemesis can possess someone if they are unwilling either. 

One thing to consider is that Justine/Nemesis has a wizard ally capable of summoning Outsiders....  Listen could be that person.  He's a Starborn....  Perhaps that's why he betrayed Ethniu, because he's an agent of Nemesis.  Morgan was worried that Harry, as a Starborn was an "agent of Nemesis".

You don't have to be a star born to summon an Outsider, at least I don't think you do.  Was Madge a star born? 

Nemesis wants to end reality.  After it sent Thomas to kill Etri, it was only a matter of time before Harry figured out Justine was the culprit.  Physically being rid of him for a couple hours was fine at that point.
"Does Lara know?" isn't that far out there (with the caveat that it was a Nemesis screw up in actuality).  It's understandable that Lara would come to mind for Justine as someone she worked with daily and who she knows loves Thomas as well.

Perhaps, but that was the sentence that tips Harry off that something isn't right about Justine. He says it at the apartment when she first says it and later on the boat, it was a tip off.  So while you may think it is understandable, Harry's gut reaction was that there was something not quite right about it.  Perhaps it wasn't so much what she said but how she said it.

So you're saying that Justine had an episode of insanity and was tricked by Nemesis into summoning outsiders?  Sorry, I don't buy that either.

No, her brain had already been taken over, she wasn't tricked, she'd already contrived to get pregnant by that time and proceed to push Thomas into committing murder.  Are you saying that
she was still sweet normal Justine at that point?  Because she wasn't.

Outsiders want to end reality.  They don't particularly care how it comes about.  "Not their style" isn't convincing.

Go back and read what He Who Walks Beside says, yes, they want to end reality, but they want to do it slowly from with in..  They are kind of like a cat playing with a mouse, it could end it quickly but it isn't.. It revels in the rot from with in.

they also kidnapped a bunch of low level practitioners, likely to brainwash them into service (though, perhaps to harvest resources.. or make kraken for some reason)

Yes, and lets not forget that is what Lord Raith did, he managed to recruit and brainwash Madge in order to summon Outsiders.  She was a low level practitioner.

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."

In PT the cornerhounds served as a way to delay Eb and Harry having their final confrontation, as a way to tease out starborn information, and as a mid-book action scene to ramp up tension.  For the plot though, the cornerhound attack doesn't really fit any character's motivations well.

But it does, Justine has every reason to either distract or be rid of Harry.  Actually it is the reader who is distracted because of the peace talks and the Battle of Chicago, both are the real distractions, while Nemesis makes it's moves.  You said there must be limits to what a mortal can call up at any time otherwise it makes no sense or it is bad story telling.  However I think you are mistaken, if the Enemy is smart, it isn't going to reveal itself willy nilly..