Transitting NRT DL/UA or HKG on UA

So my friend wants to visit me in SIN and asks a good qn...

DL fly to NRT and then down to SIN
UA fly to NRT and/or HKG and down to SIN
KE fly to ICN and down to SIN

What happens at the transit stop for these carriers?

• Will you have to clear customs and re-check with regular pax for the second sector? (doubt one would even have time, so implies an overnight stay at transit stop)

• Will they let you transit like a normal pax and onload/offload you at the gate?

• Will they not accept you at origin if you won't get on to final destination? *

* The only airline I know to do this is QF from JFK-LAX, they won't accept you in JFK if they can't accept you from LAX-MEL/SYD/BNE/AKL . This is more due to traffic rights than anything else I hear. That being said I can travel ADL-MEL domestically on CX... but this post isn't about the freedoms of the air, so I digress...

Any advice from UA/DL people who have done this at NRT or HKG would be appreciated!

KE is their best option as they fly ex DFW, but I think they only had ID90 with them, and ZED with DL and UA

Nick--there is new DL service DTW/HKG if that works anywhere in your equation. Starts early June--flights arrive HKG 7pm if that's important

I don't have the date but there is "new" DL service [back] to ICN (MSP or DTW?)