How likely is it that I or my luggage miss my connecting flight at NRT? Does Japan Airlines usually hold back the outbound flight to wait for connecting passengers?
With the connecting flight confirmed as JL 62 (AA 8408) from NRT to LAX, it can be concluded that missing the connection is highly unlikely. I have flown this route from Southeast Asia to North America — and similar ones — a number of times starting in Singapore, Bangkok, Hong Kong, or Taipei.
In general, Japan Airlines runs a tight ship and a one-hour connection in Narita is common. If all goes as designed, and it almost certainly will, you barely will enter the airport. There will be a person with a sign from the gangplank telling you which direction to go, there will be a security scan (the queue will make you nervous, but it will go quickly enough), and finally you will head to your next flight.
Recently, JL 62 has been delayed many times; it has departed late 18 times in the last 30 days. This even improves your odds of making the connection. It very well may be that the airline is delaying departures to make sure that everyone on a single ticket from Japan Airlines or a codeshare partner makes the connection despite extra time needed for documentation checks, etc.
If I or my luggage does indeed miss my connecting flight at NRT, who
is responsible for getting me to LAS - the ticketing airline or
Try not to worry. Japan Airlines would make sure you get on a subsequent flight.
Entry for foreign tourists into Japan is currently not allowed due to
COVID, but if everything goes well this is okay, because I will never
leave the airport. However, if I miss my connecting flight at NRT,
what would happen?
Unfortunately, you would be required to hang around in the departure terminal (airside) until your replacement flight. According to the Narita website, you currently can't hang out overnight airside, so it isn't clear what would happen if there were an extreme delay, but it is highly unlikely that you would be allowed to go to a quarantine hotel without a visa to enter Japan. Japan is a by the book kind of place and it isn't permitted by current regulations.
In early 2021, I had a scheduled seventeen hour overnight layover in Tokyo Haneda en route to Bangkok where I was confined to the then empty departure terminal with little more than a Pokémon vending machine for company. Those were strange days; with any luck you won't have to have a similar experience now.