Edit default applications to open KML file?
To my knowledge, an app has to register for that -- so it's not simply a question of configuration. You might want to try some other apps which are able to handle KML files, e.g. Locus Maps. If it registers itself for this file type, it then should appear in the selection, and you can make it the default app.

As it turned out, this was exactly what happened with Maps With Me, so that's one candidate working for KML files.