I recommend Codex: Daemons for a counterpoint. The Realm of Chaos is a place where everything has happened, will happen, and never happens. Slaanesh has existed as long as the warp, never been born, and was born. In short, the warp has no sense of causality.
This is the crux of the warp/realspace problem as they really shouldn't interact. Daemons become almost sympathetic when you imagine every ship with a gellar field showing up in your backyard all at once or that causality may be a narcotic to entities forced to exist in all possible states simultaneously. I know I'd pop a psyker open every so often if it meant I could exist without simultaneously not existing at the same time.
Also of note, the Emperor is just one phase Captain Uberman's existence. Previously he just prodded humanity from the shadows trying to get it to stop sucking so much.