I've been reading Blood of Elves for the first time, and honestly, her being the main character would be pretty much in line with how this first book has gone. I've been surprised by how central she is in this book, even being a point of view type character for large portions rather than Geralt. And since I assume the first season will be largely taken from Blood of Elves, since it's the first full length novel in the series, she may very well have a larger role than Geralt, at least in the first season. Don't know how the other books are.
Also, more news on the casting of Ciri. New information, which was posted on the Witcher subreddit, states that they are currently auditioning white actresses in poland, which obviously suggests that the rumor that they are looking exclusively for BAME actors for the role is false.
I imagine they will have some stuff from the short stories in there in some form, maybe flashbacks, or maybe they'll combine them in some way. But I think the showrunner has hinted that Blood of Elves will be the focus. Could be wrong, though.