1)
What was the purpose of the letter?
1.
Malbim: People should fear Daniel's G-d. The other gods have no life. They are natural forces without life or intellect, e.g. the Persians served fire. Other forces overpower them, e.g. water extinguishes fire. They are not constant; every natural power has an end. Nothing opposes G-d's kingship; His rule is eternal.
2.
Malbim (29, citing Mahari): This was an introduction to his desire and vow that he and Koresh made to send volunteers to build Beis Hashem. Therefore, he told of Hashem's great deeds and wonders, so people will fear Elokei Yisrael.
