I'm not saying the overtone series is just a coincidence, I'm saying it's a consequence of simple ratio intervals and their physics. It's not the cause of V-I harmony. V-I harmony is also somewhat a consequence of simple ratio intervals, i.e. they have a related root cause. So... they are not unrelated. They are correlated, perhaps.
Also, as an aside, tones with symmetrical waveforms (e.g. square wave, triangle wave, clarinet) are missing every even harmonic (octave, two octaves, perfect 21st, etc...). This isn't to say they don't blend well with the missing harmonics (I did say earlier that harmony still works even with sine waves), just there are some basic timbre types that are missing harmonics in a regular way like this.
I think the idea of octave equivalence is entirely learned. I do not believe this is a "natural" perception. It is nearly ubiquitous, but only because it is a very practical musical tool. The unison is the simplest interval, the octave the second simplest. A man and a woman singing together will quickly discover how easy and extremely consonant an octave is. From here it's easy to extend this idea to a practice of mostly octave equivalence, sure, but I conject that the perception itself that E4 is somehow "equal" to E5 is something you learn.
If you must deal with orchestration, the non-equivalence of octaves becomes very clear as you try different chord spacings, different bass notes, there are many places where you cannot accept it (Brahms' symphonies are a good place to look for examples of the profound impact of octave choice- he's a master of getting variety from the strings in this way). Another fun thing to try is to work with a tuning system that has no octave (look up Bohlen-Pierce, perhaps, and play with it). Writing music without the safety net of an octave can be mind-expanding.
As for scriabin being a "crazy person", he was actually quite mentally ill. He believed he was God, and that he must bring about the end of humanity. He thought he was going to accomplish this with a work he called the Mysterium (which was unfinished at the time of his death). He was known to be extremely paranoid and nervous. He's a rather strange fellow, though it's been a long while since I was reading biographies on him and I've forgotten a lot, but there were a lot of interesting things to say about his life.
Also, Ant1, 20hz isn't the lowest pitch you can hear, it's merely the lowest sine frequency. Lower tones can be heard, just your ear won't pick up the fundamental of it. If it was only a sine wave you'd hear nothing, but a saw wave, for example, has enough harmonics that your ear/brain can piece it together as a pitch despite the lack of the fundamental sine component.