Major Spoilers for all Expansions - Some general things I didn't understand

[Eisi]

it's just one of the many mechanics that exist purely for gameplay reasons, lore need not apply (aetherytes in Elpis, Eorzean gil as a multiversal currency, etc.)

1. Aetherytes in Elpis I 100% take as proof that they used Aetherytes in the Ancient world, along with the one in dream Amaurot. Why else would they get their own special model? Could have just as easily made something that's not an aetheryte.

2. Gil as multiversal currency I assume doesn't refer to the First where they went into detail on why Gil is the accepted currency there, which is actually an indicator that they take the lore more seriously than just having stuff exist purely for gameplay reasons (much like the new custom delivery line where spontaneously flying creatures [mounts] are researched). Instead you probably mean Ultima Thule, which only has been given shape through the sacrifice of Thancred, which made it breathable, traversible, tangible, communicable and I would assume also allowed for trade and stuff. Also we have the likely original Omicron currency as a counter as well: Omnicoin.

By the way, they obviously use it for gameplay reasons first and the lore follows that, rather than making the world reflect preexisting lore. It's still lore though.