Hola! The best way to make money with plat would be:
1) Crate Keys (generally only sold during events)
2) Awakening Kits (though I would wait until amulets awakenings are released so prices sky rocket)

If you don't know your email to the other accounts I doubt you will be able to get your in-game names. You say you saved them during the beta, if so they may be being used by someone else as last year in September STS released a whole bunch of inactive names. Best of Luck!