Or Xopher instead of Christopher and Xian instead of Christian?

Per the title: Yes, because "X" is the Greek letter "Chi", which is the first letter of the Greek "christos," because "Christ" isn't an English word. Jesus' whole name is Greek, Jesus Christ means "Joshua, the Anointed One." "X" is short for "Χριστός" but you can certainly write "Χριστόpher" and shorten it to Xopher if you really want to. It's the nature of language to shorten things as much as possible as long as it's understandable. You can shorten Christopher to "Xφ" (Chi-phi) if you really want to, lots of people write their signatures in some kind of shorthand.