There is general agreement among scholars that Daniel's revelations are actually vaticinia ex eventu or prophecies after the event
Nope. Not good enough, NVL. The book of Daniel was spoken of in Jewish works that were written in the 2nd century such as the Maccabees. It was also in the Septuagint which began to be translated in the third century bc. Also, fragments of Daniel were found among the Dead See Scrolls, which are dated around 100 BC.
Some, like you said, date the book in the 2nd century, but this cannot be the case either. Belshazzar was the ruler of Babylon when Babylon fell. Herodotus, Zenophon, and Berossus (5th,4th,and 3rd centuries respectively) did not know about the existance of Belshazzar at that time. If Daniel was written in the 2nd century, how would he have knowledge about this when earlier authors did not? Also, there were the prophecies fulfilled AFTER the 2nd century.