Thinking about Elidibus

[Iscah]

At this point either the Source or at least one shard is in a roughly medieval-era age of heroism, and he possesses a guy that looks like the FFI WoL, and plays hero to both feed himself and manipulate the world.

This is your inference, not stated fact, and I think you may have it wrong.

From memory, Elidibus thinks on the idea of the Warrior of Light and remarks that they were a sundered soul. There is never any mention of him taking that form before.

It's more likely that it was an early incarnation of Azem's soul who started the legend of the Warrior of Light, and Elidibus's plan in Shadowbringers is to intercept the hero-worship still afforded to this lasting concept of heroism and salvation by taking on that form and claiming the title.