Even if エレハモニカ is read as Erehamonika, the Modified Hepburn Romanisation guidelines (the one we use for osu!) clearly state that if a word is written in katakana, its roman transcription should be the spelling of the original word (e.g. https://osu.ppy.sh/s/621776 オルゴール -> Orgel (german iirc)). So yeah, unless you have an official reference saying it should be Erehamonika, you have to change the Romanisation :'(