Primal confusion - spoilers

[Cilia]

What's the question exactly?

Primals are, according to Igeyorhm, "the embodiment of mortal desire." They are, essentially, ideas given sentience and a corporeal form that sustain themselves with aether.

EDIT

On Ramuh
"... the embodiment of mortal desire." The sylphs, unlike every other beast tribe, are not belligerent in the slightest. They're a few fries short of a Happy Meal by human standards, but they're kind and helpful. Ramuh's major personality trait, the thing that drives him (like Garuda's lust for destruction and Ifrit's desire to dominate everyone) is a desire for reconciliation - hence why tempered sylphs sometimes kidnap un-tempered ones, and Ramuh is willing to give you a chance at proving mortal goodness.

Even then, Ramuh still embodies the worst traits of the sylph - he passes harsh judgment on all mortals because of Garlean incursions, and forces you to defend your ideals and the world with your life. Not too different from most primals, the only thing different is that he isn't aggressive and gracefully accepts his defeat because he knows being manifested on the physical plane isn't good for the world.

On the Ease of Summoning
Whatever Midgardsormr was doing in and around Silvertear Lake for God knows how long, it was keeping a great reservoir of aether dammed up (I think). It was still possible to summon primals before his "death," but it was much harder. So to answer the question, it was Midgardsormr.

On Primals in the Lifestream
It's kind of vague? Far as I can figure a primal only exists (in the Lifestream or otherwise) if someone acknowledges them. In other words without someone to acknowledge their existence, then the primal too will disappear from the Lifestream without someone to give it a basis for a personality or consciousness. Not saying much since Midgardsormr and the Ascians have been around since time immemorial, but even so.

Or maybe they just go Ozma. /shrug