While I still side with the "Pure Mortals lack supernatural aspects" camp in general, I disagree that a character can only take on the Changeling template at character creation. I think it's entirely reasonable and within the spirit and rules of the game for a character to enter play as a Pure Mortal, then 'discover' his heritage in play (probably involving the character tapping into Fae potential in a moment of stress). Before this point, the character was technically a Changeling, but with his Fae potential completely untouched could legitimately be counted as a Pure Mortal.
What's the difference, then, between a Pure Mortal who might someday uncover latent Changeling blood and a Changeling with -0 refresh worth of powers? The High Concept is the difference. The Changeling, even with no powers whatsoever can call upon his Changeling High Concept to do perform amazing feats. A character who's (admittedly dull) high concept was "Janitor" might have trouble lifting the heavy beam trapping his friend, but if he was a "Janitor with Troll blood", the situation changes. At the very least, the character could count on an extra +2 by invoking his high concept, but further he has a solid justification to borrow Inhuman Strength as a temporary power or even simply add it permanently to his sheet.
Or, to put it another way, "The pie isn't an apple pie as soon as the crust is formed; it becomes an apple pie only after the crust is filled and it's baked". :p
As for WCVs, my take is that if all WCVs started life with the WC Virgin template, then 100% of them would become fully blooded WCV within minutes, hours, or at most days after birth as they killed their human parent out of desperate hunger. I would play it that they are Pure Mortal at least until puberty, then swap out to the WC Virgin template and gain the WCV version of the Choice.