Ugh, wrote a response and lost it. Let's try to rewrite it...

Thank you for the response, good ser! It makes sense - although I have a few comments.

One is that I don't think the Garleans are in contention with the Primals over ceruleum as much as over aether itself. Was it ever actually stated that ceruleum is concentrated aether or an aether source? I know that the Primals are aethervorous (it's a word now!), and that Gaius at least considers this an existential threat to all Hydaelyn and uses this to justify the Garlean assault.

Secondly, I do remember the revelation that the Spoken Primal exists - and I think Siren or Ramuh would be the best possible options for that barring an extensive redesign, since they're the most 'human-looking' and primals always seem to look somewhat like their chosen people.

Also, "Odin (or Ramuh, but probably Odin given as we've seen concept art for him) is stuck in Dalamud" has been a theory of mine for a very long time now. I feel like - given the obvious reference to FF12's Judges with their armour - the Garleans sincerely want to remove the influence of the Primals from the sapient races' doings. (At least, Gaius does. I can't wait to see him in ARR :3) Your theory that Midgardsormr was actually -protecting- Eorzea from the Primals that are now running rampant all over it is -incredibly- interesting and a rather obvious solution that I didn't think of myself. /doff hat.

Grah, now you've got me all thinking about this again. Hopefully the Eleventh will give us some answers...