When flying carriers use a term called most significant carrier. If that's DL then DL rules apply and they only give D1 allowance if in D1. If you happen to get upgraded on the domestic flight doesn't matter. DOT allows MSC instead of first flight because of the fact the allowance international is more then domestic most cases. However you're stuck with either first flight or MSC and the airline can choose which set to follow. They however must stick with whichever they pick throughout the trip.