Na the mirror is an aspect of reality, or between reality I might say. It's one reason why I think Mirror Mirror is called such when he pulls Harry into his reality. It's through a metaphysical mirror. Imo everybody has that mirror, it's very indicative of the gateway that exists inside us that our souls would pass through in certain mythos.
The mirror seen in Thomas's soul probably has nothing to do with other realities, because it's in
his soul.
And the real reason it's called Mirror Mirror is that's the title of the Star Trek episode that introduced Beard Spock. I'm sure there'll be an in-universe reason for it as well, and it would make sense thematically if mirrors (already established in the series as gateways) are in fact how he's pulled through, but I seriously doubt the image in Thomas's soul has anything to do with it.
I'm guessing that she used Thomas to create the binding she used on the King. Certainly Thomas supplied the power to keep it active after her death. Say that he was the vector. She couldn't attack directly so she attacked Raith through his living male heir. Raith may have been aware of this avenue of attack since he always killed his male heirs. If you're enamored of the mirror metaphor, say she attacked Raith's demon through the mirror. As long as that connection to the demon side was alive in Thomas she could bind the demon even if she couldn't kill Raith. The idea is simple and it's probably wrong. But I like it anyway.
Eh, it's probably just because he doesn't want anyone who can usurp him.
edit
I wonder if Malcolm was a fae halfling who chose to be human. Maybe a son of Lea. Goofy idea, but I throw it out there. It creates a connection between Lea and Margaret and would go a long ways towards explaining Lea's affection for Harry.
Because Margaret
La Fay, famous for traveling the Ways in
Faerie for more than 100 years, couldn't have met Lea without the man that she just met introducing them?
And Lea knowing Margaret isn't enough explanation for Lea's affection toward Harry? Considering Lea explicitly references her relationship to Margaret whenever it comes up, and almost never Malcolm?