When was everything going well?
Well, she talks about not being in total control of her hunger at the talks, well before she gives most of her energy to Thomas. She should be well-fed then and therefore not have any control issues.
TC makes kind of a big deal about how some of the Whampires are highly controlled (like Lara) and some aren't (like Madeline).
It also might have to do with her being sweet on Harry. Susan couldn't separate her "hungers" for Harry, and Thomas can't with Justine.
This is possible... but that also seems new. Has Lara developed feelings for Harry since the last time we saw them interacting? If so, is that natural, or is it like the Luccio situation in SmF/TC?
But here is the thing, even if she owes Lara favors, why would Mab jeopardize her Accords or the talks?
Oh, I don't think she really did. I think Mab is "repaying" the favors in a way that benefits Mab/Winter.
I don't think the Accords are really being jeopardized. By my understanding, if Mab loans Harry to Lara to repay a favor, Harry is then acting as Lara's agent. So the White Court, not Winter, would be responsible to the svartalves under the Accords. Does Mab care if the White Court and the svartalves go to war? The White Council/Red Court war didn't seem to trouble her.
The talks, maybe, but I doubt the svartalves really have the power to ruin the talks. They could walk out, but everyone else could still come to an agreement. And the svartalves have already rejected an agreement with the Fomor (in "Bombshells") so I don't think they really have much negotiating power in this case. The svartalves are super-crafters, but not a very militant power.
So I don't think Mab is really risking all that much, and I think she already knew what Lara was going to do with the favors. (And I kind of think she arranged for Thomas to be put in a place where Harry could rescue him.)