If you book in advance and your plane is delayed by a whole day, your ticket is void. That's it. You're done.
...In which case you can just show up to the station and buy an unreserved ticket and ride, like everybody else and like I mentioned is the better idea.
My point is that reserving in advance has no purpose because of reasons you're giving to somehow justify it. If you don't book in advance, there is zero chance of missing your train due to a delayed flight since you're buying it 5 minutes before riding.
You seem very, very confused.
For a overseas tourist arriving by flight, anyway it's better to avoid the three weeks. I believe it's on May, August, and end of the year, but former two is Japanese local holidays so you can easily avoid. Then you don't always need reservation shinkansen because free sheets are available (but YMMV).
Which is, of course, when you're statistically the most likely to be traveling.
And the fact that a ticket is valid the whole day doesn't help much when your flight is delayed overnight or by a day.
Some people tend to travel precisely when others are not, and might not run into this problem, seems like that might be your case. Other people travel precisely when others are too, and will run into it all the time. As far as China, every hard sleeper I ever took was sold out in advance, zero holidays.
As far as I can tell, you're entirely confirming what I'm saying.