That first baseman is a complete moron.
So long as Baez doesn't get to first, it doesn't matter when/if the runner crosses home plate. AT ALL. As soon as Baez started running backwards, turn around and touch first base. End of inning, no run scored.
If you decide to chase him down the line, fine. Just walk up and tag him. Sit on the fucking baseline if you want and wait for him to come to you an hour after the runner crosses home plate. ANYTHING besides trying to get the runner at home. End of inning, no run scored so long as Baez doesn't get safely to first base.