Rambam’s issur seems only to be for a woman to be the queen regnant over the Jewish nation not merely the queen consort. If so, that means Esther wasn’t even a problematic question regarding Rambam’s issur since only Achashveiros was regnant and Esther only consort.
Though it does raise the question how the Jewish nation did in fact have a queen regnant on occasion. Perhaps those instances were violations of Jewish law.