And the problem with someone who wants to convert to Judaism, but doesn't accept Rabbinic law, is that-- they are not then practicing Judaism, and they are not Jewish
I don't dispute that, although the acceptance of Mishnah is unquestionably an act of faith - we believe it to have been written and passed down orally accurately.
I dispute the degree in which this is enforced, and the apparant double standard applied to prospective converts vs secular Jews. A person born of a Jewish mother is a Jew even if he cooks and eats lobster on Shabbat. Is defiance of Jewish law the same as rejection of Jewish law?
There really is a great arbitrary nature to all this. If you find a beit din of Sabbath observant Jews, 2 witnesses and a rabbi, to convert you, you are a Jew. Certainly they have an obligation to ensure your sincerity, but no 2 beit din's will maintain the same requirements, and there is essentially no way to ensure observance thereafter.