You already know where I stand on this. Haruka cannot possibly be Misty.
Ash and Misty part ways in Viridian City. Ash heads south to Pallet Town and promptly sets out for Houen. Misty heads north to Cerulean, deals with a rogue Gyarados and a League inspector, and manages to arrive in Houen in time to ride her bike from her home there to Mishiro Town and watch Ash's ship enter the harbor? I don't think so. Not to mention Haruka's own backstory, which is being established solidly enough to deny the possibility outright.
Furthermore: Misty is an expert trainer. Haruka has no clue what to do with a pokemon, and it shows.