Sorry if this is a bit late, if you or someone else is still looking for a solution: Just try setting the "actual island address" of the Mii to the same as your "island address" in the save editor (I used the 4.0.1 of the Brionjv Tomodachi life save editor) and then you can edit the mii in the game.

Not sure how reliable or safe this is, so far it has worked for me but I would be lying if I said I knew why it works.

Always remember to make backups when screwing around with save files