The pashut pshat reason that Hashem is not mentioned in Esther is that the Megillah was originally an official document of the Persian government and the Persian government wasn’t about to endorse the Jewish religion.
That is why the references to Hashem are veiled (“revach v’hatzoloh yamod layehudim m’mokom acher), Achashverosh is portrayed as not realizing that Haman was trying to kill the Jews, and the Megillah ends with a statement about how wonderful Achashverosh was.
BTW the Torah and Tanach often round off large numbers (and even not so large). Dibra Torah k’loshon b’nei adam.