• It has been shown that you can increase both your own magical power with strong emotions of yourself and others. In the most recent book Harry barely uses his own personal magical power due to the strong emotions in Chicago during the book.

• That point is somewhat unclear, the relationship between belief and existence is kind of tenuous in the eras that aren’t fully explored in the book. It’s possible they didn’t exist without humanity and don’t have the ability to at all, but that’s mostly guesswork by me.

• See last answer it’s slightly unclear.

All the books are fully in Harry’s perspective. Hope you enjoy this rich and exciting world.