Found a partial bit for what I was saying, how Giancola was considered to be the MAlign operative or agent in Haven territory.
Mission of Honor, Ch 34 wrote:"You made the point that we don't know what whoever hit Manticore's ultimate objectives may be, but we have to suspect Manpower's involved, for all the reasons you enumerated. And then we have Cachat's suspicion that Manpower was involved in the attempt on Queen Berry from which it's only a short step to their being involved with Admiral Webster's assassination in Old Chicago. For which"—her eyes bored suddenly into Theisman's—"some form of suicidal compulsion appears to have been used. Very much, now that I think about it. like what happened to a certain Yves Grosclaude."
It was suddenly very, very quiet.
"Are you suggesting Manpower was working with Giancola?" LePic asked very carefully.
"No, I'm suggesting Arnold was working with Manpower ," Pritchart replied grimly. "If they're willing—and able—to manipulate the Solarian League into going to war with the Manties, why in the world wouldn't they figure they could do the same with us? I mean, look how much easier it would be, given the fact that we didn't even have a formal peace treaty from our last war!"
-snip-
"I know. And the only thing more dangerous than not seeing conspiracies that are there is seeing ones that aren't ," Pritchart acknowledged. "But talking about conspiracies and suicidal assassins, there's that attempt on Alexander-Harrington, too. We know we didn't do it, although I've never blamed the Manties for figuring we were the ones with the best motive. But if Manpower's been moving chess pieces around like this, and if they have the technology—or whatever—they used to control the assassin who killed Webster and that poor patsy who carried out the Torch attack, why shouldn't they have tried to pick off one of the Manties' best military commanders? Especially if the object of the exercise was for us to trash Manticore for them?"
Mission of Honor, Ch 40 wrote:"Which brings us back to Nouveau Paris," Pritchart said grimly.
The others looked at her, and she barked a metallic, snarling laugh.
"Of course it does! For that matter, Tom, you and I have already discussed this, in a way. If McBryde was telling the truth about the existence of this 'assassination nanotech' of theirs, I think we finally know what happened to Yves Grosclaude, don't we?" She showed her teeth, and this time the glare at the backs of her eyes burned like a topaz balefire. "Frankly, it ties in rather neatly with the only bits and pieces of forensic evidence Kevin and Inspector Abrioux managed to come up with at the time. And just why, do you think, was this 'Mesan Alignment' kind enough to provide Arnold with one of its most closely held, top secret toys? Remember what you were just saying about sleepers, Tom? And that little comment of yours, Denis, about producing movement economically?
-snipped one paragraph0
"It makes sense, doesn't it?" she pressed. "They played us— me— by having Arnold doctor the diplomatic correspondence. Hell, they may've had someone at the other end doing the same thing for High Ridge! No one's seen hide nor hair of Descroix ever since the wheels came off, now have they? And then, when we figured out what Arnold had done, they played Elizabeth by convincing her we'd killed Webster and tried to kill her niece exactly the same way the Legislaturalists killed her father and Saint-Just tried to kill her! God only knows how many millions of civilians and spacers—ours and the Manties'—these . . . people
As far as Haven thought or believed, prior to Cachat re-appearing with Simoes, they already believed that Giancola was working for Manpower. Once Cachat, Zilwikki and Simoes reappeared, Haven simply changed it from Manpower to Mesan Alignment, and then brought the whole thing to Manticore.
During the later meeting, when Detweiler finds out about Simoes reappearing, and Haven's beliefs, is when he mentions that regret of providing Nesbit with the tool for Giancola to assassinate Grosclaude. I just haven't found the specific passages just yet. I'll edit in when I find it.
Edit: Found it
A Rising Thunder, Ch 14 wrote:“Oh, it gets better, Father,” Benjamin said harshly. “I don’t know how much information McBryde actually handed Zilwicki and Cachat, or how much substantiation they’ve got for it, but they got one hell of a lot more than we’d want them to have! They’re talking about virus-based nanotech assassinations, the streak drive, and the spider drive, and they’re naming names about something called ‘the Mesan Alignment.’ In fact, they’re busy telling the Manty Parliament—and, I’m sure, the Havenite Congress and all the rest of the fucking galaxy!—all about the Mesan plan to conquer the known universe. In fact, you’ll be astonished to know that Secretary of State Arnold Giancola was in the nefarious Alignment’s pay when he deliberately maneuvered Haven back into shooting at the Manties!”
“What?” Albrecht blinked in surprise. “We didn’t have anything to do with that!”
“Of course not. But fair’s fair; we did know he was fiddling the correspondence. Only after the fact, maybe, when he enlisted Nesbitt to help cover his tracks, but we did know. And apparently giving Nesbitt the nanotech to get rid of Grosclaude was a tactical error. It sounds like Usher got at least a sniff of it, and even if he hadn’t, the similarities between Grosclaude’s suicide and the Webster assassination—and the attempt on Harrington—are pretty obvious once someone starts looking. So the theory is that if we’re the only ones with the nanotech, and if Giancola used nanotech to get rid of Grosclaude, he must’ve been working for us all along. At least they don’t seem to have put Nesbitt into the middle of it all—yet, anyway—but their reconstruction actually makes sense, given what they think they know at this point.”
So the MAlign knew what Giancola was doing, and they didn't have a thing to do with him manipulating the war. But Nesbitt is something that isn't quite a spy, but isn't inside the onion either. So exactly what Nesbitt is, in relation to Mesa is... ambiguous.