[Jim Porter]

I'm going to change my answer.

The ball was indeed dead the moment the pitcher failed to complete the delivery of the throw to first.

Jim Evans puts it this way:

"When a pitcher balks, one of three contingencies may occur: (1) the pitcher stops his delivery and retains possession of the ball, (2) the pitcher continues his delivery and completes his throw to a base, or (3) the pitcher continues his delivery and completes a pitch to the batter."

Since the pitcher did not complete delivery of his initial throw, the ball is dead at that moment, and the balk is enforced.

Even though the failure to complete the throw was the balk, the pitcher still retained possession and did not complete the throw. That means that the attempt to throw to second was a second delivery, the ball should have been ruled dead at that time, and the balk penalty enforced.

Interesting situation.