Here are my thoughts:

1) For definition from NW, Taiwan is not part of China. So M class is upgradable from US to TPE. At this point, Chinese government has not filed a protest

2) 25,000 miles will upgrade you all the way to TPE. If available, you will be in FC from mainland to Hawaii, then business class to TPE, if your layover is less than 24 hours in HNL.

3) On the return, if the layover is more than 24 hours, you probably still get upgraded using 25,000 miles. My understanding is that one way includes layovers (someone corrects me if I am wrong).

4) It is possible to get Q one direction and M fares other direction, but that happens rarely.

Good luck.

A warning: WBC upgrades to Asia are VERY VERY difficult to obtain, especially you are on M fare. If you bought a B fare ticket, then it will be a little easier.