There is no one official name for the band. Various official sources have romanised it as Koenji Hyakkei, Koenjihyakkei, KoenjiHyakkei, Koenji-Hyakkei and others. The last.fm site rules explicitly say that names in foreign languages should be in their native character set and Koenjihyakkei is unquestionably Japanese so 高円寺百景 is the correct tag. It's also worth noting that their name is written as 高円寺百景 on several of their album covers as well (such as their first three studio albums and Live at Star Pine's Cafe).
Already done, but the way last.fm's corrections system works means that it'll probably never happen because whoever uploaded their music to last.fm uploaded it tagged as Koenjihyakkei. It's the same reason all of Spotify's mistaggings remain in the database - too many people have played the wrong tags for tag corrections to take effect, even if MusicBrainz and other sources say they should be something else. There's basically no chance of the tags being corrected until MusicBrainz integration is worked into last.fm (if that ever even happens), so we're unfortunately likely to be left with Koenjihyakkei split over two tags.
We need term for all j-zeuhl, not only for Yoshida's projects. F.e., [artist]Happy Family[/artist] are Japanese zeuhl indeed, but you can't call them "yoshida-core" because Yoshida never played with them, AFAIK.