The question I'd ask is as Catapult said if Midgarsormr was sealed, was it in the sky? It appears above the Agrias in the Battle of Silvertear Falls and is coming straight down. If it had been sealed in Mor Dhona there would have been a bit more Lore (I assume) told about it, such as merchants from Ul'dah traveling to Ishgard and chancing upon a huge seal. If it's sealed in the sky, then what the heck is above the clouds and does the Crystal Tower reach it?

But I agree Anonymoose, that's a good concept about Sharlayans coming to Eorzea to found Sharlayan from Sharlayan. You might have just answered the question we've had for Ferne for him with a very easy response.