I'd rule that you complete the ability on the card, then the insteading triggers at the insteader's discretion, then undo whatever would be undone by the insteading if applicable, then Dominants can be played. That both stops slapjacking, follows the rules of abilities completing, avoids situations of "I wanted to do this, no, I wanted to do this," and fosters a faster playing environment without making speed of play a physical factor that can determine game outcomes.