Rosh Hashanah 3: Jewish Kings, Non-Jewish Kings, and One Who Was Both (not quite)

Counting the years for kings... How does the mishnah know that the kings' rule is counted from Rosh Chodesh Nisan?The Gemara tracks through biblical verses... Twice. Once, via a beraita. Plus, the fact that non-Jewish kings' rule is counted via Tishrei, and why that is. But what about Koresh (Cyrus)? He's a non-Jewish king whose rule seems to have been counted according to Nisan. Ah, but he was good! Or was he? Plus, the Gemara conflates him, Daryavesh (Darius), and Artaxerxes as one - which may solve the contradictions between records of Persian history and the Jewish tradition. Also: the rabbinic approach of "conservation of biblical personality."