I was thinking that Sirleck never got around to possessing either awakened or mages, perhaps sticking to people with money, a nice outward sign of a successful host. He might simply have never gotten around to looking for a magic-user who might actually have greater than normal magic resistance. And Ellen, who is magical in origin, does have an advantage in magic resistance as has been pointed out.
In my headcanon, it's just because she is a bad ass. Cause you know, Bad Assery is a separate stat you can have, and comes into play at special dramatic moments.
Well, we know that the child of two mages, according to Pandora, is almost certainly another mage, but we don't know what the child of a mage and a Tulougol Seyunolu will be. We don't even know for sure how Grace can procreate (perhaps two different ways?) or if she inherited Mr. Guyur's ability to create eggs without a mate. Heck, for all we know, if Grace and Tedd want to ensure a mage child, they might be able to include DNA from a third donor, or it might be moot (might turn out that the child of a mage and a Tulougol Seyunolu will be mage always). Or the child might be more like Grace, who is herself a very powerful, important person.