I'm really struggling with trying to figure out what song KB might be wise to cover from the year she was born. Maybe Why Can't I by Liz Phair could work for her or Are You Happy Now? by Michelle Branch. Stuck or More to Life by Stacie Orrico could be interesting songs for her to possibly cover as well. I also wouldn't be shocked if she's the one that ends up covering an Evanescence song instead of Abi, although I'm really not sure she has the voice for it. If she decides to cover My Immortal, she's really going to have to bring it. As far as Kayko is concerned, I think Absolutely (Story of a Girl) by Nine Days would be absolutely perfect for him to cover! I think a slowed down version of Kryptonite by 3 Doors Down, Teenage Dirtbag by Wheatus, Broadway by the Goo Goo Dolls or Wonderful by Everclear could also work for him, although it might not be a bad idea for him to sing a rearranged version of a pop song, like Lucky by Britney Spears or Gotta Tell You by Samantha Mumba. And perhaps Complicated by Avril Lavigne or Don't Know Why by Norah Jones could work for Julia.
I'm not gonna debate who had the better performance, but on the numbers, I think It's just a matter of taste. Minivan usually likes her types of singers more than the ones you mentioned.