ORDnHKG

I had just booked a trip on UA ORD-NRT-HKG, of course as everyone knows NRT-HKG is on NH(NQ) codeshare, seats cannot select when I make the reservation on UA, but after I purchase it (on UA website), it automatically assign me the seats on both segment for NRT-HKG-NRT. And I have no idea why both segments I got 31D on the 763.

I called UA, and the agent is able to change it to aisle instead 34F. I asked if it is possible to get the seats closer to the front, was told as UA codeshare, UA only purchase the seats between row 31 to 35 in Y on the 763, any other seats can only assign at the gate. I am also able to ask the agent to put me on request for window.

Therefore, to your answer, yes, UA is able to assign the seats on NH, they can't do that only if those rows of seats that UA had purchased had already filled up, but other Y seats are still available for purchase. And NH for sure cannot access any PNR code from UA, even you are on that NH flight.