+1 for UA, especially since it's so easy to get UA miles via Chase UR points if you want to accelerate your earnings.
Can I earn miles directly on my UA account if I am flying NH, since they are both *A? Are there any disadvantages (fewer miles earned?) of doing so?
Check out the United chart for earning miles on ANA flights
here. As you can see, it all depends on the booking code your tickets are issued under. But virtually all flights for your purposes (trans-pacific and within Japan) earn at least 100% on UA, with all of the J class fares earning 175% (though only 150% of that will be preferred qualifying miles -- that is, counting toward elite status).
Your connections from Japan to China will earn at the same rate in J, and at a slightly lower rate in Y, depending on booking code.