How likely will I or my luggage miss a tight Japan Airlines connecting flight in NRT while travelling from Southeast Asia to the US?

I've completed that trip, and had an un-ordinary experience.

The night before my trip, I looked at FlightAware to check how often my flight was delayed, and, lo and behold, my flight (i.e. the one for the next day) was now scheduled to depart at 10:00 am, which is two hours later than the original departure (as per the email sent by American Airlines at the time of booking) of 8:00 am. I was unable to check my booking on the Japan Airlines website, but could do so on the American Airlines website - and I saw that they rebooked my NRT->LAX and LAX->LAS legs onto later flights, giving me a 1h 30min connection in NRT and a (longish) 5h 15min at LAX.

I didn't get any email from either of the airlines at any point in time informing me of the change in schedule. When I turned up at Changi at 8 am (instead of 6 am), I found the check-in queue to be fairly empty (presumably because many people thought the flight would leave at 8 am). They told me that they delay was because of crew-related reasons (apparently one of the cabin crew members was sick). They printed my boarding passes for all three legs (for the updated flights which I saw on the American Airlines website) and handed me a free packed breakfast as an apology for the delay.

I later found that the NRT->LAX flight that I was on, while not the same one as my original flight, was delayed for about an hour or so, presumably so that the passengers from my SIN->NRT flight would still be able to make it to the US without too much delay. It also appeared that the NRT->LAX flight I was on was the only connection to the US for those from my SIN->NRT flight that day - there were some people who originally had an SIN->NRT->SFO itinerary that ended up on my flight to LAX (i.e. they ended up flying SIN->NRT->LAX->SFO).

So, in summary:

1. It's possible that the SIN->NRT flight gets delayed so that you miss your connecting flight to the US.
2. If you miss your connecting flight, they will try to put you on another flight to the US without that much of a wait in NRT.
3. You might end up entering the US via a different airport than planned, which may cause you to need to take an additional internal flight within the US.
4. Japan Airlines will give you a free meal when your flight is delayed.
5. Check your ticketing airline's website when nearing the flight date to see if there are any planned changes to your itinerary.

How likely is it that I or my luggage miss my connecting flight at NRT?

You have a very good chance of making it, otherwise they wouldn't have sold you the ticket. A missed connection (passenger or bag) incurs significant cost for the airline so they know quite well that a "almost safe" connection time is. Some airlines go down to as little as 25 minutes (Austrian Airlines in Vienna). One hour isn't particular aggressive if you don't have to do deal with immigration and customs.

Does Japan Airlines usually hold back the outbound flight to wait for connecting passengers?

Unknown. Airlines policies do change over time and circumstances and they are not published. United used to be brutal: I once had a delayed incoming but still made it to the gate with maybe 2 minutes to spare (I can run fast). My boarding pass beeped red and I was told "Sorry Mr. L, we didn't think you would make it so we already gave your seat away". They have mellowed out recently but it's still hard to predict.

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 operating airline

Technically the "responsible" airline is on the hook, which in this case would be Japan. Chances are that One World has a process for this in place, so both AA or JAL would be able to handle it.

Entry for foreign tourists into Japan is currently not allowed ...

Not sure about this specific case, but there is typically a process in place. If you need to stay overnight, they would probably cart you to a quarantine hotel, where you can stay but are not allowed to leave the premises. I've seen that for cases where a passengers got stranded but didn't have documentation to enter (re-enter) the country.