Question?

[Angrboda]

What's the difference between a brain experiencing these things and one influenced by a chemical to believe it is? How would you recognise the difference, from the brain's perspective?

Very easily and I'm glad you asked. This answer is also for everyone one else saying it was all a drug hallucination:  It ended when I decided to reenter my body.

And it was literally like switching mediums: The OBE state was like mental air, everything was clear and I could see for miles.  When I decided to stay on the planet and reenter my body, it was like slipping back under water (literally and figuratively) I slipped back into my life/personality/dream of being "me" and the clarity was gone.

What drug goes from full strength experience to zero, when you decided it does?

Anyone who's ever been in a car accident and experienced the slowing of time that occurs when danger is detected knows that the brain can react with swift speed, altering the way we experience reality. To answer your question, the altered clarity might have just been one aspect of the experience and when the brain changed that aspect, the change to experience was global. You might still have been under the influence of altered brain state, only without the 'clarity epiphenomenon'. I'm not suggesting a drug that has global effects like alcohol; something more subtle, affecting different systems in predictable ways. (NDEs, whatever they are, appear to form some common core behavior of the brain that is shared by people undergoing life threatening experiences.) That it was compelling to you is part of the experience, that it's compelling. Who knows why the clarity ended at the moment you had a desire to return to your body; perhaps you've just confabulated that it did. You wouldn't know. Just as in dreams things can seem clear that are befuddling upon awakening, the NDE may imbue us with a false sense of clarity and coherence. Again, how would you know if it did?