Two things must be acknowledged about the Book of Daniel:
(1) Its composite structure.
(2) The fact that it is not unique in this feature.
Many O.T. and even N.T. books have a composite structure.
Genesis for example is actually a collection of nine 'booklets', or ancient Diary Entries, either by contemporary eyewitnesses (early sections), or generational historians (later sections). As it accumulates, it becomes a tribal and finally a national history to the time of Moses. Its highly doubtful Moses wrote it himself. Rather the tribes have in Genesis surrendered a shared historical account.
Both Kings and Chronicles openly acknowledge using previous written records, currently now lost.
Even prophetic books, such as Isaiah, contain block-sections that have apparently been inserted from historical accounts, in order to provide context and meaning to Isaiah's original prophecies. Isaiah chapters 36-37 for instance appear to be an early version of or have been borrowed and edited by the compiler of Kings. (Kings 18:13-19:37)
The parallel sections of Kings, Chronicles and Isaiah
form a kind of simplified O.T. Synoptic Problem:
2Kings 18:13 = Isaiah 36:1
2Kings 18:14-16 = / 2Chronicles 32:2-8
2Kings 18:17-37 = Isaiah 36:2-22 / 2Chronicles 32:9-19
2Kings 19:1-5 = Isaiah 37:1-4 / 2Chronicles 32:20
2Kings 19:6,7 = Isaiah 37:6,7
2Kings 19:8-19 = Isaiah 37:8-20 / 2Chronicles 32:17
2Kings 19:20-37 = Isaiah 37:21-38 / 2Chronicles 32:21
2Kings 20:1-11 = Isaiah 38 / 2Chronicles 32:24
2Kings 20:12-19 = Isaiah 39:1-8
Likewise, it is acknowledged by Luke himself and commentators, that Luke and also Matthew have used Mark as a base for their own fuller gospel story.
Both of these books treat their source material in blocks.
Many other authentic and historical books from about the same period,
also include and incorporate documents by others, even in other languages.
Thus we have Ezra-Nehemiah in its final form being merged into a single scroll,
and inside those books we find quotations from Persian rulers in Aramaic.
Similarly, the Book of Esther references court records and/or laws and even conversations by non-Jewish persons involved in the events described.
So it should be no surprise that in its final form,
the book of Daniel represents an organized collection of stories about Daniel,
along with written records by Foreign Rulers, and also Daniel's own writings.
It should be recognized without serious difficulty, that such collections were
originally produced for many prophets by their companions, assistants, helpers, and contemporary historians.
This is obviously the case for Elijah (and his servant and successor Elisha),
Jeremiah and his scribe Baruch, and the followers of Isaiah, who organized
and collected the stories and sayings of these great prophets.
In Daniel's case, we may surmise that such records and collections were made and preserved by Daniel's companions, and Jewish organizers such as Ezra and Nehemiah.
When we turn to the contents of Daniel, we can see a simple and expected structure, along with features that can be reasonably explained, even when
they are unique in their degree or extent.
The Structure of Daniel has remarkable, but not inexplicable features,
given its circumstance and the era in which it was produced:
To begin with, the first, and then the 9th to 12th chapters are written in Biblical Hebrew similar to Chronicles and Ezra-Nehemiah. The middle chapters are written in Aramaic quite contemporary with the Babylonian and Persian periods.
The 1st chapter serves as an introduction and provides the historical setting for the book. The last chapters claim to be final prophetic chapters by Daniel himself (more on that later). The middle (Aramaic) sections form the bulk of the stories about Daniel as a court minister with the Babylonians and Persians.
This core of the book (chapters 2-7) seems to be the original collection,
completely written in Aramaic, and it forms a remarkable Chiastic Structure,
indicating it is all of a single piece and composition.
In contrast, the remaining Hebrew portions of the book show no awareness
or at least concern at all for the Chiastic structure, and simply offer a sequential set of three more dream-prophecies.
the entire book as a unit are in fact worthless.
The biggest criticisms, namely that the 'author' is only aware of and
mostly concerned about the period under the Greeks,
especially Antiochus (c. 167 BC), only really apply to one chapter,
namely Chapter 11.
But this was probably the very last section (chapters 10-12 are a unit)
added to the book, and as a unit it is quite different than all other parts.
The strongest criticism therefore would most effectively suggest
that the last three chapters were added, and that the rest of the book
must in that case be much older.
We have a similar situation with the Book of Enoch, where
demonstrably foreign and later sections were added,
in the simplest manner, namely tagging them on on the end of the original.
Although Chapter 12 is of interest since it mentions the resurrection,
several other parts of the Aramaic core also reference this common belief.