I would rather the Osaka tag remain blank if the alias is undone with a wiki that directs to the various uses of the name. Given that Osaka city tag only has 6 images in comparison to 481 for Kasuga "Osaka" Ayumu, giving the city preference doesn't seem right either. As the discrepancy between the two is so large though, I don't see why simply having a note under the Kasuga Ayumu wiki that guides users to the other use of "Osaka" wouldn't work out.
It should be noted that it seems a lot of the images on pixiv also appear to tag her "Osaka" and do not even bother using her real name, so having Osaka_(Azumanga) and Osaka_(Azumanga_Daioh) aliased to Kasuga Ayumu would be valuable. Users unfamiliar with the series may attempt tagging the character with one of those names, regardless of whether the current Osaka alias is in place.
Should be noted a similar issue with naming occurs with Tokyo (artist) and Tokyo_(city).
As for the code names, I agree with what Log has already said. Leave the code name to the wiki and use the real name for the tag. Unless of course there is some reason to use the code name for the tag, such as the real name being a spoiler or perhaps the real name being unused by the character (like with Hime from Kaibutsu Oujo). Also mentioned don't use the code name in the wiki if that is also a spoiler (such as identifying "Tri-edge" in .hack//G.U.).