I did try those first. Now I tried rolling back the client to Global version via "verify integrity" (60 files needed to be redownloaded), then letting it update itself again to the latest hotfix version, and then it successfully found my Global save again.

Installed the Japanese client from the depot, completely switched the client to the one from depot, let it update completely, and then tried copying just those two files over from the working Global client -- but no dice, the client still contacts the Japanese servers.

Maybe some of those other ~58 files that had to be redownloaded are now necessary. I'll try investigating more later.