you set your mind to understand

"you determined to understand the vision"

your words were heard

This can be expressed in active form. AT: "God heard your words" (See: rc://en/ta/man/translate/figs-activepassive)

Here this refers to a spirit who has authority over a human nation. AT: "spirit prince"

This probably refers to the various kings who ruled over nations in the Persian Empire, and who had to obey the king of Persia.

"Michael, on of the chief angels" or "Michael, one of the archangels"