$3470 on UA from BNA (via NRT on ANA in both directions; note that BKK-NRT is not a lie-flat):
https://goo.gl/flights/cngV

I had to move the return to the morning of 12/21 instead of post-midnight to get that price.

Agree on $2778 from LAX on Air China (CA), but you need to figure in the extra cost of getting to/from Los Angeles. Also, if you make the purchase on one ticket, and there's a delay, the carrier that was late has a responsibility to get you to your final destination. If you use separate tickets, they do not.