QIf the follower I selected with this follower's Fanfare ability leaves the field, does this follower's attack become 0?

ANo. The value added by this follower's Fanfare ability will not change, even if the selected follower leaves the field or has their attack value changed later.

QIf the attack value of the follower I selected with this follower's Fanfare ability has been changed, does this follower's attack become the same value?

AYes. This follower gains attack equal to the selected follower's current attack value.

QCan I select a follower from any player's field with this follower's Fanfare ability?

AYes, you can.