Maharet is cool as cucumber. You really think she would allow anyone to kill her protego? Heck no. As soon as David hears that Merida is bonded to Maharet, it would send him into a mediocre panic. It’s his sire-sister, he knows just what she’s capable of just as much as Maharet knows that David craves power, by any means he gets it. She knows of his plans for human enslavement, and has never agreed with it. She also has backed the Romanus clan for many millennia; that’s not going to stop any time soon. She doesn’t get involved in the day to day issues and trusts Marius to run things, but this isn’t an ordinary issue. This is Merida.
To Maharet, she couldn’t care less about a newborn so Carrie isn’t a concern to her at all. For her, it’s all about her protego. She sees Merida as the carrier to her genetics and her Elementalist power. Like Cherry, who has the gene but dormant. Cherry has no interest in awaking it, but Merida came to her so eager to learn and improve. It played right into her ego. And while she admits that the Razele line may have some unique powers of its own, it is not a comparison to the Elementalist power. David is always been jealous of it because it is not replicable in any way; you either have the gene or you do not. Period. It is also why David fears the Twins so grievously. Maharet might be able to just walk in there, sit down and demand he release Merida and David will probably do it to avoid her and her sister’s wrath. But she would leave Carrie to rot. That is Va’lis’ problem, not hers.