Xander is Jack Carter

Xander took on the name of Jack Carter as his alias for when he wanted to pretend to be normal.  He's obviously actually a US Marshall, so maybe that's what demon-fighting falls under or he liaises or something (to be figured out).  And he gets married, has Zoe, and gets divorced.  Then he gets sent to Eureka, which is the same old thing except with science instead of magic and demons.  Eventually a supernatural problem comes up, and he solves it easily while everybody else is still panicking over violations of the laws of physics.  Nobody in Eureka knew about his other life, even Zoe.