What is the explanation of God being in front of the worshiper's face, and is this why it is forbidden to pass in front of a worshiper?

A worshiper must not spit in the direction of the Qibla because Allah Almighty is in front of the worshiper's face. He is truly established upon His Throne, yet at the same time, He is truly in front of the worshiper's face in a manner befitting His Majesty. Allah Almighty encompasses everything and faces every worshiper with His Countenance.

As for preventing passage in front of a worshiper, it has no relation to Allah being in front of the worshiper's face. Rather, the intent is to prevent the passage of Satan, to help the worshiper focus their heart on their prayer, and to avoid distraction.