Any info on the relationship between The Demon Lord of Hags and Witches and Baba Yaga? I'd think a Demon Lord known as the "Mother of Witches" would be pissed if there were a Mortal running around using the same title.
Unless of course Baba Yaga is more powerful than Mestama.
Baba Yaga's CR 30. In all likelyhood, she actually IS stronger than Mestama.
Other than Kotchtchie, Baba Yaga's on cordial terms with the archdukes of Hell, the demon lords of the Abyss, and other various powers. She isn't interested in their holdings or their power, and they're generally aware that Baba Yaga is catastrophically dangerous and treat her as a peer.
In all likelyhood, Baba Yaga simply doesn't care about Mestama's claim to the title, and Mestama's wise enough not to push the issue with Baba Yaga.
I can picture Baba Yaga and Mestema getting together and swapping stories over tea and cookies, and by tea and cookies I mean unspeakable foodstuffs created with the Cook People hex.
I could see Mestama resorting to "catty" acts of vengeance against Baba Yaga. Such as ordering her followers to kill any Jadwiga they find or to work to destabalize Irrisan. But she wouldn't know that Baba Yaga doesn't care about either enough to give it a moments notice.
If anything, Mestama would want to be the state religion of Irrisen (which already worships Zon-Kuthon and and Lamashtu).
She'd have more to gain from actively seeking converts among the Jadwiga than trying to randomly kill them. One way, she's just killing them. The other way, Mestama gets their souls!
Baba Yaga wouldn't give a damn, either. If her descendants are stupid enough to worship a demon lord, then they'll get what they deserve.