Sure. I've come across some. A lot of them play with the idea of magic systems. Since both universes have their own unique magic, fanfic writers often explore how they would interact. For example, in one story I read, Yennefer was trying to understand the spells used at Hogwarts and compare them to her own magic. Another one had Harry Potter learning some of Yennefer's more complex and dangerous spells. You just need to dig a bit to find the ones that suit your taste.
There are. Some fanfics are really creative. They might create a new plot where Yennefer has a role in the fight against Voldemort. Or it could be a story about a relationship between a Harry Potter character and Yennefer. It could be a friendship or something more romantic. The great thing about fanfic is that the possibilities are endless. Just search for the relevant tags on fanfiction platforms and you'll start to discover some interesting reads.
Yes, there are quite a few. You can start by looking on fanfiction websites like Archive of Our Own. There, you'll find various authors who have combined the worlds of Harry Potter and The Witcher (where Yennefer is from). Some fanfics might focus on a crossover adventure where Yennefer visits Hogwarts, or perhaps a story where a Harry Potter character ends up in the Witcher universe and meets Yennefer. It really depends on what kind of story you're interested in.