Also, JL and NH checking staff seem to have no issues with these separate ticket transit (of course at outstations, ymmv)

If you fly via another airline (eg BA LHR-NRT to connect to JL NRT-BKK), you may not find the same experience at BA checkin and they'll deny you boarding. All ymmv

Any possible way to overnight in NRT T1
 

I currently have a ticket booked that has an overnight connection in NRT;

12/6 arrival @ 3:00PM in to NRT T1 NH011 ORD-NRT
12/7 departure @ 11:45AM from NRT T1 TG643 NRT-BKK

I just read above that there is 'no overnight stays in NRT' Any suggestions on what to do, other than cancel and rebook?

I was able to change my LAX-BKK-LAX flights from a much less nail-biting 1 hour layover on 2 PNRs to a 16.5 hour overnight at HND on the way there, and an 8 hour non-overnight layover at NRT on the way back. Anyone have recent experience to which restaurants are open between HND (arriving at around 7pm on the 26th, leaving at noon the 27th for the flight to BKK, and then NRT arriving on 12/31 at 9am then leaving at 5pm?

I have not looked recently, but the Haneda website has a pretty detailed listing of what is open and when. Do expect anything but the vending machines to close around the last flight of the day.

Arriving on JFK-HND connecting onwards to HND-FRA 6 hours later. Both from T3 I believe. Is it possible to transfer airside to T2 for the new lounges?

The international section of T2 and the new lounge is closed. No indication on when it will reopen.

I have a 9 hour layover at HND coming up (waiting to go to BKK). Since I can't leave the terminal and apparently can't go to the transit hotel what can I do to rest / recuperate and pass the time? Are there any showers available? Any recommended restaurants and/or lounges?

Depending on the airline you fly and whether you have lounge access, ANA has the suite lounge open and JAL has their first class lounge open. Some changes and reductions to services compared to the normal first class levels. Both lounges have showers. The ANA lounge is also a priority pass lounge at the moment.

The F&B outlets are open airside, I never really looked at them, so I have no recommendations. But Haneda's homepage do list options and opening hours.

Dear experts -- is international transit through HND allowed just like for NRT? I see flights offered for travel from US to Thailand later this season, connecting through HND. However, the IATA travel pages (https://www.iatatravelcentre.com/world.php) only specify transit rules for NRT and are silent on whether HND is ok.
Thanks!

You can transit through Haneda no sweat.

Spent 6h at HND yesterday, connecting from JFK-HND to HND-FRA. ANA Suite Lounge is open for F/C passengers. The rest of the airport is more or less dead. Everything is closed and there are practically no passengers. Felt like a ghost town. Showers are available as normal in the lounge.

United/ANA transfer between NRT and HND
 

United is trying to sell me a ticket with a 6hr transfer between NRT and HND to BKK. My understanding is that this transfer is not yet allowed. This would be substantially cheaper if I can make it work but seems unlikely from what I have read. Can anyone please confirm this is/is not yet allowed?

Thanks in advance!

So it looks like the Royal Park Transit hotel is operating again, at least from Dec onwards? Just went onto their website and was able to bring up some test bookings.

Would appreciate any advice on whether this itinerary seems feasible in the time of Covid:

20th January 2022: SYD - HND JAL J, Arrive Haneda 7:15pm. Cash ticket.
Stay overnight at Royal Park Transit Hotel
21st January 2022: HND - LHR JAL F, Depart Haneda 11:30am. Separate ticket, award flight booked with Qantas points.

EDIT: Just called the Royal Park hotel - they said that the hotel is only open on the "Japan side" and I was advised it cannot be used in January as a transit airport.

A transit between NRT and HND is not allowed.

did you book this as a single ticket? I'm thinking of doing a similar trip JFK-HND and HND-CDG but I can't get JAL to ticket it on a single ticket