[Jim Porter]

The rule requiring the pitcher to throw to an occupied base has nothing to do with the fielder. It's only about runners. With a runner on first only, a pick-off attempt to third would be a balk because no runner holds legal title to third. Third would be unoccupied, so the pick-off attempt would be to an unoccupied base, and therefore a balk. There isn't any more mystery involved than that.

Once again, a pitcher is required to throw to first base. It is the only base he is required to throw to. He can fake to other occupied bases, or he can throw to a fielder stationed away from second or third, as long as second or third is occupied.

Any first baseman must be in the vicinity of the base in a position to make a play in order to avoid a balk call on this rule. It isn't necessary that the first baseman be right at the base. The only thing that matters is that the throw is to the base. We can use the first baseman's position to decide whether or not the throw is to the base.