Vidqun,
If one insists that Daniel was an historical figure of the 6th century BC as described:
Arriving in Nebuchadnezzar's court and immediately assuming duties similar to a prime minister, for one, it makes him a partner in the destruction of the Temple and overseeing the sundry details of the deportations. While he might not be able to stomach Babylonian court food, it appears that there was considerably more in his duties that he could choke down.
His recollections, writing in 3rd person in chapter 3, appears to include a number of Persian officials ( satraps) in King Nebuchadnezzar's court.
Your grammatical argument ( remain, reside etc. into the period of the first year of the reign of Cyrus) in 3rd person authorship, still does not address how it can be the first year of Cyrus's reign (555 BC) or how he can be writing testimonies several years afterward.
None of what you say seems to address the biggest holes in the argument about "Darius the Mede":
Why would a Mede king have a Persian name?
How do you explain the references of Thucydides and Herodotus?
The Peloponnesian War: Book 1, Paragraph 19:
"Not many years after the deposition of the tyrants, the battle of Marathon was fought between the Medes and the Athenians....So that the whole period from the Median war to this, with some peaceful intervals.."
Thucydides makes at least 50 references to the Persian invaders as Medes - and the Battle of Marathon was 490BC. Darius (the Mede) was king of Persia at that time, not in 538 BC.
Here is a section from Herodotus describing the later invasion of Greece under Xerxes:
[7.136] And afterwards, when they were come to Susa into the king's presence, and the guards ordered them to fall down and do obeisance, and went so far as to use force to compel them, they refused, and said they would never do any such thing, even were their heads thrust down to the ground; for it was not their custom to worship men, and they had not come to Persia for that purpose. So they fought off the ceremony; and having done so, addressed the king in words much like the following:-
"O king of the Medes! the Lacedaemonians have sent us hither, in the place of those heralds of thine who were slain in Sparta, to make atonement to thee on their account."
Then Xerxes answered with true greatness of soul "that he would not act like the Lacedaemonians, who, by killing the heralds, had broken the laws which all men hold in common. As he had blamed such conduct in them, he would never be guilty of it himself. And besides, he did not wish, by putting the two men to death, to free the Lacedaemonians from the stain of their former outrage." Book 7 - section 136.
While Herodotus probably was off by an order of magnitude in describing the invasion fleet of Xerxes, he identifies him much the same as Thucydides, but more explicitly - as a Mede or as a Persian interchangeable with such an identity. Herodotus also relates a complicated story of the origin of Cyrus as the son of Astyages - and Josephus morphs it into Darius.
Ahasuerus is the Hebrew derived form of the same monarch known among Greeks as Xerxes, Khshayārshā in the Persian inscriptions.
Consequently, a writer who speaks of "Darius the Mede" conquering Babylon has his historical memory scrambled, filtered no doubt by obtaining ancient history via an education influenced by the 2nd century BC Hellenic occupation.