As far as I know... we have no idea! Elder usually says older, better, stronger - so let's go with that! One thing I've noticed is their placement on the elemental wheel. Odin and Bahamut appear to be astral and umbral - for all we know, one of them is a freakin' "good" guy.
That would put quite a spin on things, yeah? Pretty much the core definition of a primal (aside from my snarky definition of "starts cults, gives pretentious monologues") is that they're from the void and require such gifts of aetheric energy. Who's to say where they'd get it otherwise? Darnus basically just willed aether to Bahamut (oversimplified, yes), so I'd say this idea is up for grabs, lol.
This hasn't been explained at all. My personal theory is they reach out in non-physical form to start with - voices, ideas, metaphysical changes, gifts of power and knowledge, something. The more the newborn cult takes hold of these things and returns the favor with aether, the more the situation escalates until you get full blown Garuda throwing entrails everywhere and screeching. I'm betting that the "Hear... Feel... Think" and the Echo is exactly that ... we're getting the very first experiences of how they latch onto a society. Who is it? ... SIREN! (Kidding, kidding. We've beaten that dead horse to a pulp - but if it's right I expect everyone to mail me one gil. Just one.)
Were they banished? Maybe! Before The Twelve, it's possible that this process ran rampant and was later expelled by "better" gods. This is all speculation, but it fits the Greek influence the game is based on very well (i.e. the twelve Olympians overthrowing the Titans).
This one MIIIIIGHT not be true. We don't know for sure, but I currently believe that he didn't truly physically manifest until Darnus force-fed him aether from ERRRRYTHING. Errything from crystals (Mor Dhona, Toto-rak, Xel...however you spell that), to the dead primals themselves. I think his fresh manifestation is part of what let him break free.