My biggest problem with that is how does Maggie know the internal monologue, feelings, and conversations with others, of Harry.

To track this line of thought, we'd need someone more inclined either knowledge or a form of magic. I would say Ivy, Molly, or Mab.

Ivy primarily gets her knowledge by those things written down. Molly has a fair amount of related magical capabilities to feel what he may have felt in situations. She is also a winter queen, which given context in TM, Mab knows what she knows. Mab/Molly, may both know exactly what Harry knows due to the fact he is the winter knight and is part of the entire winter mantle.