Oh, ok, now I get what you point out as speculation. It's relevancy is lost on me though.
OK, I'll explain as best I can. Note that this is nothing but speculation... but that's all we ever do here anyways.
The DF was presumably created by an entity in its universe (and no, we don't know which one did it, though I suspect Jim does
, existing as what is called a multiverse. The multiverse is presently being fueled by mortal decisions, in which each decision splits off a new branch, creating billions upon billions of new worlds.
But if Ferrovax had that capability at one point, and Ferrovax isn't a free willed being, how did we switch from a non-free willed entity having control over the creationary aspects of the DF to what we have now (i.e. the universe being driven and populated by mortal free-will)?
That's where the origins theory ties in. Both modes of creation are represented in the text: when Mother Winter (who does not have Free Will) imposes her will on reality, it creates a specific set of circumstances that sets boundaries for how things can and cannot exist. Yet when a free-willed being exerts his capabilities on that same reality, it can (but does not always) create a new set of boundaries, in some cases completely reversing the original context by which reality behaves.
So why have the Fae evolved separately from mortals who have free will, yet still retain a link through a tiny piece of mortality?
Because both modes have merit when it comes to doing one thing: creating reality. Mortal free will seems to be a little more potent in terms of its creation, simply due to its nature- it can and does make decisions that often contradict one another, rather than synchronize to a single idea or way of behavior (as we've seen with the majority of non-free willed beings.) The Fae are the only ones we see who have demonstrated the capability to act in both manners- making free willed choices (changelings) and making only a single choice, predicated upon previous modes of existence (those like Mother Winter and presumably Ferrovax.)
The Fae are truly the only beings in the DV who have demonstrated a capability to evolve all three ways, into changelings, Fae, and complete mortals. There's a fluidity (forgive the term, I don't have a better word to hand at the moment) in their base nature that doesn't exist in any other species.
The white court are, perhaps, the closest to the fae (closer than even humanity) as a merger between an immortal demon and a free-willed mortal. But even then, once they have 'chosen' their path, they cannot reverse it. The Fae can and do change theirs, and often, based on their allegiances and current circumstances. Their origins, by the fluidity in their nature, do point to their having emerged from a similar substance as humanity- but not necessarily the same substance.
Can you kind of see where I'm going with this? There's a link between the decision or reality-making capability of an entity, and its origins.
My theory does not exclude free willed beings causing his power to shift to another being that was already immortal, just that some free willed beings ascended by taking on power like Fero's. It hypothesizes that most of the immortal beings with that kind of power used to be mortal, but explicitly states that there are some that this does not seem to apply to. It also hypothesizes a possible different genesis origin for the being Fero as well.
Perhaps, but I have to point to creatures like Mab, who despite the ascendency have kept small pieces of their mortality. It would seem to point to the Fae's retained link to humanity being important in terms of their origins, rather than describing the overall scope of the creatures in the DV.