The dragon domain is one of her main due to the fact she is the only one with dragons as part of their domain still in existence. The other dragon gods are dead, which means Mordred is now stuck as their primary god. And being the only god of dragons with it just being one of her minor domains would be an enormous blow to dragons (ego) everywhere so it became a main domain to prevent dragons from gathering together to rebel against her.
Also I've come to a decision on Cathy, please consider it to be Mr never appearing in this fic. The most it will ever matter in this story is that it does some trade with the old world but that's it. When it finally deigns to exist in lore as more than a rumor, I might consider letting it exist as more but I hope to be done or at least close to finished with this story by the time Warhammer 3 is supposed to come out (2022) which is when Cathay is supposed to come into existence.
Hmm, a tough question since my understanding of High Magic and what exactly the average user is capable of is very vague since my sources are mainly Teclis, Mazdamundi, and Kroak. I'd guess that she is slightly below the average high elf that specializes in high magic. She can do absurd stuff and can learn magic incredibly quickly compared to normal people because she is immune to miscasts, she still does miscast but it has no effect on her and she can do whatever she wants with magic with no consequences to herself.
That is before she became the goddess of magic, while she has no access to anything Lizardman(Because Slann are absurd), Elf(because they have their own gods), Dwarf(because they also have their own gods), or Chaos(because Tzeentch), she does have access to all non-Chaos human knowledge of spell craft. If a human knows the magical theory, then she knows it, if a human can do it, she can do it too. While this doesn't mean much when it comes to high magic because humans can't do that, it does directly make her better at weaving magic by a decent amount as she reaches the pinnacle of current human skill and knowledge instantly.
It would be, definitely. Heres to hoping someone makes an omake of it and shares it with us.