I too fly NH/UA multiple times a year and your list is the best summary I've seen thus far. Your observations are spot on - I'd like to provide my experiences on (4) above.

If booked as a UA 016 ticket AND the NH segments are UA codeshares (e.g. UAxxxx), then NH cannot assign the seats. If you pull up your booking on NH's website, no seat selection is possible. Similarly, NH agents cannot assign seats and will tell you that it is UA that must assign seats.

However, I've found that UA agents in the US may not familiar with this. You need to call UA Japan to effectively do this. The crux of the issue is while UA cannot see seat maps, they can "send a message" for a seat assignment, which if available, is confirmed fairly quickly, usually in seconds. US-based agents see there is no seat map and assume no seat assignments can be made. UA Japan-based agents, however, do this regularly.

In a nutshell, you have to figure out available seats (pull up NH seat map through dummy booking), call UA Japan and provide them desired seat numbers.