I am working on booking SEA-PVG-NRT-SEA as well. I want to use GPUs and get miles so need to stick to UA. I found that an RT to NRT married with an award RT NRT-PVG was the most cost effective at W fare.
I cant book until I get next years GPUs though as I dont want to buy unless I can confirm the upgrades right away.
SEA-XXX-PVG PVG-NRT(award) NRT-SEA is an alternate option but prices a tad more.