If you are booked one-way SEA-NRT-ICN on UA, you will be safe (and UA875 is my favorite international UA route, so I hope they treat you as well as they treat me) departing at NRT and not continuing on to ICN.

Japanese Passport Control and Immigration did not ask me if I had a return ticket, they just asked me how long I was staying in Japan and the purpose of my visit. So they should not ask for your ticket.

As for your return ticket, if it is a one-way coming home, you are safe, even if it ends up on UA, much less another Star carrier (NH) or a non-Star carrier (CX).

Only if the return segments are on the same PNR as your outbound will UA possibly cancel it, even if it is on another carrier (be it within or without Star).

If UA can issue the ticket for those carriers on their stock, I'd be worried. Mind you, I could be completely off-base here, so ask for a second or third opinion, please.