1) The key is what the Gemara tells us that she abandoned the idols of her father's house. That was a tremendous act of rebellion against her entire upbringing, where Avodah Zarah was the philosophy of everyone. She cast away the idols and entirely changed her life. Of course, this all happened before we received the Torah, so we did not yet possess the 613 Mitzvos. In those days, the simple understanding is that conversion only involved rejection of the all-prevalent idol belief and worship.

We always suspect that a potential convert may have an ulterior motive for conversion. This is why the Gemara (Menachos 44a) tells us that when a woman applies to convert we suggest to her that her real motive might be that she wants to marry a Jewish man. This suspicion was clearly not applicable for Bisyah.

2) Hash-m criticised Moshe for receiving the Erev Rav as Gerim. He said to Moshe, "Your people have acted corruptly" (Shemos 32:7). Rashi writes that Hash-m did not call them "the people" but rather "your people" because the Erev Rav that Moshe accepted as converts, without consulting Hash-m, are the ones who have persecuted the rest of Bnei Yisrael.

Kol Tuv,

Dovid Bloom