Ok, here is another view.
Why are we assuming Louisoix kicked the proverbial bucket? All we saw was a white light descending as Bahamut charged a megaflare. We have been explicitly told there were final events that occured after we were whisked away - ie: it wasn't over yet. And if it wasn't over, it means both parties still drew breath.
We've also been told that something occured that made Bahamut vanish. So again I ask, how can we assume Louisoix died if Bahamut's entrapment was successful?
I didn't see a noble sacrificial death. I saw a hero getting us out of the way so he wouldn't kill us upon pulling out the biggest trick in his bag, one he would have preferred not to use. Louisoix was always of the mind that we should take the easier and less world-shattering option first.
Mythology in FFIV tells us that Odin was once mortal. Explicitly, a king of Baron who became Odin. This is where I'm coming from with the transformation into Odin thing - the potentially biggest trick in Louisoix's bag, but one that turns him into that which he fights against, FFX Sin-style.
And if Odin appears for the first time in an age only after the calamity, something had to have summoned him or unleashed him - there needs to be a culprit. If that culprit (and vessel) was Lou, you could understand why he left it as his last option.
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On the subject of what primal appeared to prevent the Garlean advance, I wouldn't place Ramuh as too far-fetched. The Sylphs are fearful in the pre-calamity world of summoning him, so they must have a particular reason for this. Maybe they did summon him back then and the affair ended rather badly. And before anyone says "seal not yet broken", recall that the Sylphs are highly skilled conjurers - it is entirely possible that there is a more complex and resource-intensive way of summoning a primal that they are familiar with.
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EDIT RATHER THAN TRIPLE-POSTING:
Louisoix's Sharlayan neck tattoos were hidden by his clothing most of his time and you full well know it, Mr Moose.That forehead tattoo was something else.