Just a note regarding Ciri's age... or, well "age", as since she's a time-traveler it makes no sense to count the time from when she was born. Anyway: despite very little training, Ciri has shown absolutely exceptional magical skills.
Blowing up a shed with no training while just trying to levitate a basket? Yup.
Opening up a portal no-one had been able to use while still untrained and shitting herself with fear? Check.
Traveling through time while grievously injured, still with close to no training? Piece of cake.
Summoning fire magic and not going batshit? Been there, done that.
Ciri has not only more raw power than any sorcerer or sorceress alive, but the natural skills to both use it and not come to harm, be it through misuse or abuse. She doesn't need training. Just think about this: Ciri jumped through time once, close to death. OK, I'll chalk it up to an uncontrolled reflex due to self-preservation. Here's the thing though: she bloody well did it again, and this time she controlled it. Sure, it took her a few tries, but we're talking about a barely adolescent, untrained girl doing what no man or woman had ever done, even at the summit of their art. In half a dozen tries, she had teleported to within a hundred feet of Geralt in a snowstorm. She didn't realize he was there, but that's besides the point: she hit the nail square on its ugly head. And when the time came, she chose to teleport into another effing dimension. That shouldn't even be remotely possible.
Now, take that in, and ask yourself: knowing the common sorceress can slow down her metabolism to age at only a tenth of the normal process, how difficult would it be for Ciri to reverse her aging process if she so willed? She literally has her entire life to learn how to do it, and it took her only a fraction of an effort to master time travelling when she was but a child.
If CDPR decides to shove an Alvin-aged Ciri in our face, they can just rest on the lore. Hell, it would even make sense story-wise: they introduced Alvin as an obvious, blatant reference to Ciri. Do you think Ciri would pass on a chance to relive her childhood years, those she never enjoyed because of War and Destiny, with the only man she looks up to as her father? Where's the problem? She can age normally from eight to thirty, Geralt will barely have (physically) aged two years, as witchers do: she can still spend uncounted years of adult, mature time with him. And she'll actually have a happy childhood to reminisce about this time.
Ciri coming back as a child makes as much sense as any other theory.
P.S.: sorry if I overcomplicated things. It's just too tempting.