Sounds as if the simpler course is to stick with what we have.

We'll look for open unreserved seats on the Shinkansen, else we'll cope with AB.

We have about 30 minutes to switch at Tokyo, which should be more than enough time.

We arrive at about 17:00 for a 19:00 flight (business class check-in, no checked luggage). With luck, passport control and security won't be a mess. This would have been more than enough time for a 14:00 flight based on experience last week and we saw people with signs for upcoming flights to get you through passport quickly if necessary.