CG Midlander

[Catapult]

Interesting that you should mention the horn (key).

An identical horn is seen in the opening Ul'dahn sequence, Flowers For All. It is blown by a Mammet, which turns the captive Gobbue into a such a rage that it breaks its restraints, rampages through the festival and sets off the entire sequence of events that lead to a foray into necromancy.

This timeline just got bigger.

EDIT:

It may also be safe to assume that not all "archons" left on their journey simultaneously. Yda and Papalymo could have gone first to look into Gridania's intentions surounding the Garleans (the reason they cite during the opening quests themselves) and stumbled upon a bigger picture with the echo. Thancred could have then done some investigations around the Ul'dah religious cults with early echo use on a more recent past. And lastly, Ysh'tola could have investigated some deeper roots around Limsa.

Ysh'tola also probably has the strongest mastery of the echo: She was able to echo jump into the same moment of your past 5 times without flinching. It only just occured to me how she did that.