answers are below the photos
Markus and Alex
Carlos and Jorge
Zion and Haby
more photos here.
1. Kevin should play 18. Qh5! White threatens 19. Bxh6 Qxf6 20. Bg5+. If 18... gxf6 19. Qxh6+ Kg8 20. Bxf6.
2. Black can give white an isolated pawn by force; white gets to choose whether to keep all the pieces (6. exd4), trade a pair of knights (6. Nxd4 Nxd4 7. exd4), or trade the queens and a pair of knights (6. Nxd4 Nxd4 7. Qxd4 Qxd4 8. exd4). If you have an isolated pawn, you want to keep the pieces on the board, so it's best to take with the pawn immediately.
3. No, Bxa7 loses the bishop to ...Kb7.
4. 11. Nb5! threatens the Bd6 and to trap the queen with Rb1.
5. Xonatia should block with 13...Ne5! since 14. Nxe5 Qxa1+ 15. Kd2 Bb4+ 16. Kd3 Bf5+ 17. Be4 Qc3 is checkmate.
I'm enjoying a new book called The Invisible Gorilla: And Other Ways Our Intuitions Deceive Us by Christopher Chablis and Daniel Simon. They did a survey at the World Open a few years ago and found out that most chessplayers think they are underrated.
watch this related video.