Just posting to note what the companies currently say:
Discover currently says that in Asia, Discover is only accepted at Diners Club merchants in just 5 countries: Philippines, Singapore, South Korea, Taiwan, Thailand. (Mainland China is obviously excluded because of UnionPay, though the situation is not explicitly stated for Hong Kong and Macau.) They also say in Japan, Discover is accepted at "almost all locations you see the JCB acceptance mark", as well as at the merchants that display a dedicated Discover logo (personally seen several merchants that do so). Japan is excluded because of JCB, but more on that:
JCB has a page where it looks suggests:
- American Express card acceptance may be arranged through American Express International
or JCB
- Diners Club International card acceptance may be arranged through Citi Cards Japan
or JCB
- Discover card acceptance may be arranged through JCB only
- UnionPay card acceptance may be arranged through JCB only
So for Japan at least, it would seem the lack of Discover acceptance via Diners Club International can be attributed to Citi Cards Japan, and the inconsistent acceptance of Discover via JCB is due to merchants which do not wish to pay(?) to accept Discover.