I currently have two separate business class award flight reservations:

JL: BOS > NRT > ITM (Osaka)
JL: NRT > BOS

It's looking very unlikely that we'll be able to stay in Japan for this trip in early September, so we'd like to get a ticket (it looks like the only timing that works is Ethiopian airlines, not on the same alliance) from NRT > ICN and go to Seoul instead. I have two questions:

1. Do you think I'll have any issue ditching the NRT > ITM leg of my JL flight so that I can take the NRT > ICN one (separate reservations, separate alliance).
2. When I check in at the airport and am asked my JL what my final destination is, what do I say to them?
3. Is there any issue when transferring in NRT from one airline to another on separate reservations?