The tiebreaker rule between parents applies only if they don't agree.  So the answer to ""Can the child be the qualifying child of any other person for 2020?" is No -- because that other person has agreed that the child cannot be.  If you want to be extra careful, I suppose you could get a signed statement from the parent that "I agree that Tommy is not my qualifying child this year."

That is a worthless worksheet.  Who came up with it, anyway?  Someone who learned programming from a correspondence school?