Well, you just gave some good arguments why it WON'T be Ciri. At least in my opinion
Ciri is too obvious for me. TW3 is supposed to be about Geralt's personal journey in search of his loved ones - and that's why those loved ones should be somewhere in the distance, not taking main stage, so Geralt's story can shine.
Playing as Ciri wouldn't also benefit the storytelling. Geralt/Yennefer could be better, as they both might cooperate at some point in the game on finding their "daughter", giving different perspective on the same issue. Hell, even Geralt/Eredin would give the players more context - why is Geralt in such a hurry? Why is the Wild Hunt chasing Ciri? Those could be the angles answered with using this type of storytelling technique.
Playing as Geralt and Ciri brings much less to the table, so I definitely hope the writers decided to explore other options.