Significane of Lara Dorren Mating With Elf?

[CURE]

I'm currently playing through the first two games again and I am wondering why the whole Elder Blood scheme supposedly didn't work out. Was it explained in the books that it was important for her to mate with an elf rather than a human? Because it seems like their little program of trying to create a super-child came to fruition considering that people still want to capture Ciri, and descendants of Lara Dorren are still really damn powerful and can bend space and time. I'm just going to assume it has something to do with the prophesy that all humans will die and only elves will remain, but the child's genes are muddied by a human father so therefore the child will die or some ♥♥♥♥.

And another thing, just now many people have Elder Blood? The books in Witcher 1 lead me to believe it's some sort of common thing to find children who can travel through time, and regular mages have to train them. And if that's the case, why is Ciri so special.