Why are there pixies, etc. on the First but not the Source?

[Calysto]

dynamis couldnt get into the world enough to cause influence like that till after zod.

Wasn't the whole point of the sundering letting Dynamis have more influence by splitting aether info the shards (so we get more control over it ?)
Zod mainly protected from "external" Dynamis I think

I feel like trying to apply any level of scientific logic (even fictional science) to fae folk is at best an exercise in futility, and at worst completely missing the point of those characters.

I don't think Eo Tyr knows how to spell 'science', despite her playing a horrific game of god in her free time.

While you're not wrong about trying to apply logic to Fae, Eo Tyr is as scientific as pixies get : She knew the causes and outcomes of the change and act accordingly