No; the malk-voice was the sign of her rage. She could act calmly (icy control -- it's a Mab thing), but not quite speak calmly.
She wasn't in a rage when she was speaking through her malk, the subject matter had nothing to do with anger.. Her and "The Watcher" had a common enemy, if Harry continued to refuse to become her knight, she'd get Thomas, because being in love as he was, that made him human enough to be her knight. When she started talking in her own voice that is when all hell broke loose because she lost control...
However in the context of things the above moment while important, it is what happened after that we don't know about. As in, what did Mab do to try and help Maeve? Could she do anything to help her? We know for several months between Changes and Cold Days she was in a cave on Demonreach trying to keep Harry alive.. What happened during that time? Were those the critical months that put Maeve beyond help so her death was the only solution? If so, will Mab end up holding Harry responsible?
But we don't know how long she had known... she entered book-10 already knowing, and she spoke normally back in Book 4, but that's a loooooooooong stretch of time where she might have realized.
If she had really known, Mab would have done something about it simply because she couldn't afford to have the adversary hanging out in her court. We have all gone though events where we know something isn't quite right, but we cannot put our finger on it. Then that "eureka" or "oh shit" moment happens when your brain actually is able to put two and two together and get four.. When it is something you should have known, but didn't, you get really pissed about it, that's what happened to Mab. Even if she was angry as you say when she was talking through her malk, she wasn't pissed.. When things finally added up in her brain and she got four, she got pissed, things turned to ice and ears bled.
Later, during the scene in the chapel (the last chapter, IIRC)... honestly, I don't think we know what made her self-control slip, made her use her own voice.
We do, because Harry asks her about it in Cold Days, and she answers, "it was the knife." The knife wasn't the subject of conversation when she spoke through her malk.. However you can hold a conversation on one subject, and be thinking on another at the same time.. We've all had those kinds of conversations where we are talking, suddenly the other person exclaims, "oh shit!" Or it is you who is talking and you realize you had forgotten something or realize something unrelated to the conversation, "oh shit, that's what happened!"
Harry mentioned the assault on Arctis Tor, and named Thorned Namshiel. Mab never confirmed the assault -- nor Thorny -- as causing her rage.
No, she didn't, because it wasn't.. That isn't to say she wasn't angry about the assault, she was, her and the "the Watcher" had a common enemy.. But that didn't cause her to lose control.
She permitted Harry to assume. That -- right there -- is classic Faerie misdirection: not-lying and leaving the mortal believing something that is un-true; thinking Mab herself had confirmed it!
I don't think it was part of her calculation in that moment, and anyway there was no reason for her to direct Harry one way or the other about the attack on Arctis Tor, not during the chapel conversation in Small Favor. She simply lost it when she realized what had happened to her daughter and why.. "it was the knife." She freely admits that to Harry in Cold Days, and as Harry said,
" A few years back, you got angry. So angry that when you spoke it made people bleed from the ears. That was why. Because you figured out that the adversary had taken Maeve. And it hurt. To know that the adversary had gotten to her."
"It was the knife," Mab said.
No quibble from Mab, as straight forward an answer as you are going to get from her, "it was the knife."
The exchange is so typical of faerie deception, I figure it at far less than 50/50 chances that Thorny was the Fallen at Arctis Tor.
I have a low-key WAG that what slipped Mab's self-control was her Emissary and soon-to-be Knight being so stupid!
No, if it was about faerie deception, Mab wouldn't have lost control.. If she had thought Harry was being so stupid, she would have told him so.. Oh she was busy trying to move the goal posts a little so she could still claim a favor from Harry, i.e. become her knight, she even suggested Thomas for the job to persuade him, but she didn't lose control..