KIX service from the US is spotty. NW had a DTW-KIX non-stop, then it got cancelled, right around the DL merger. There's still a DL KIX-SEA flight. It sounds that you're doing what could be recommended... fly to the arrival city, so that the minor only has one segment as UM, and shared flights are an added bonus.

As noted above, UM service means the minor is never outside of the guardianship of either an airline employee, or an adult pickup them up or dropping them off.