Take it as a way for a character to peak through the fourth wall and speak directly to the MC. When a PC opens their brain, it's time for you, the MC, to tell them something you wanted to tell them, maybe that front that's going sour because nobody's done anything to prevent it, or that place you're eager ot see your PCs going to because you crafted those very cool custom moves you're dying to show them, that kind of things. Of course, as with everything in AW, misdirect, never tell its name and put your bloody fingerprint on it. That's for how it works, mechanically.
Thematically, it's better to define your maelstrom. I kept it vague and shit for six games and my players, especially my brainer, were uneasy with it. With some advice from the nice people here and at story-games.com, I made it the result of a failed alien communication - a message so alien that it provoked world-wide cancer, thoughts that bleed outside your brain and massive mutations - and it gave much, much flavor to the game dynamics.
BtW there's a very good thread about what people did with their Maelstrom at storygame: http://story-games.com/forums/comments.php?DiscussionID=14499
For the rest: depends a lot on what your maelstrom's like. All your options are a-good. But the maelstrom doesn't know everything, and you don't get to choose what it tells you. So yeah, secrets are possible, and even legion. E.g. in my game, thoughts bleed outside your brain and into the maelstrom if you're alone and far from home for too long. So to keep secrets from the maelstrom, you have to keep company. Of course, that means it's more difficult to keep secrets from other people you live with.
Big part of defining your maelstrom is to decide what it wants, and what it knows.