I never tried DMT, but from what I can tell, e.g. deep bhang journeys are indistinguishable from dreaming (well, one essentially sleeps), and are very similar to trances induced by shamanic drumming which feel more like lucid dreams while awake, maybe only not that deep--a matter of degree as you say (the same analogy can be drawn between hypnagogic states and classical LSD "trips").

Also, when you describe how various "molecule mixes" result in slightly different states, I can very much relate--different strains of Cannabis have definitely different kinds of "personality" too (sometimes they slightly vary even between various plants of the same strain).

As for the oxygen--no doubt there's a correlation. When you look at all studies (if I understand them correctly) the cerebral blood flow (together with brain activity) goes down when on LSD, DMT, or psilocybin.

The idea of "familiarity" is intriguing. I've never payed much attention to it, but there is definitely a certain feeling of "coming home" (if that's what you mean). I would say, it has to do with all three points you've mentioned, plus I would add saturation of our endogenous systems (serotonin, cannabinoid) and perhaps also that perception is constructed in basically the same way during waking and dreaming states (but I guess, that's your point No.1).

Anyway, a lot of fascinating ideas in your write-up.