“Among you of all his people.” This proclamation was sent out to all the people, so the meaning is, “among you [in Persia] of all his people [the Jews].” We can thus understand it as, “Whoever among the people in Persia is a Jew.”
“God.” Since this proclamation came from Cyrus the Persian who worshiped gods besides the God we know as the true God, a more accurate rendering of “God” in this verse would almost certainly be “god,” not God, especially in light of the last phrase, “he is the God [god] who is in Jerusalem.” It was common at that time to believe that there were different gods in different locations, and since many of the so-called “gods” were actually demons, that was true to a large extent. Of course, that was not true of the true God, but Cyrus would likely not have thought that way, as this phrase indicates (see 1 Kings 20:28).