Well I'm sure if they were able to Echo thousands and thousands of years they would see things before the Allagan Empire. But I say the Echo only works on people and objects with things they've actually experienced. They'd have to physically enter the Crystal Tower to Echo anything that has to do with the Allagan Empire. I think it's more so they found the Truth of the Echo, many people being religious (like our Path Companion), and it made them have psychological breakdowns that everything they've been taught or believed is wrong.
Minfilia: Of those few who have...not one is willing to speak of what he saw at the end of the Path. It is as if they all bore witness to a truth too terrible...or too shameful to be told.
Primals awakening, Empire truth and lies, The Twelve and possibly the Life of Eorzea. It could all be too much to handle in a vision and they just shutdown.

I think we can assume that both events happened around the same time, which would mean it's 2 separate Horns. 2 Horns that came from the same beast which is why there aren't 3? Travanchet doesn't believe we should have the power of the Horn so no way he sold it. We know Travanchet took the Horn and is either an Ascian or affiliated with them, the Ul'dah Horn that Corguevais took is MIA, I believe the Lala Twins caught up with him but I couldn't tell you what happened to the Horn.