Dear Ann,
I respect your position re: the date of composition for the Book of Daniel. As to historical problems, I see (at lest) two problems: (1) The identity/historicity of "Darius the Mede", and (2) the historical information provided in Dan 11:40-45. As to other problems, I think we (with a little bit of special pleading) can provide acceptable answers.
As to the Biblical canon, I think we should realise that it is somewhat anachronistic to speak about a "canon" before, say, the second or third century CE. But, obviously, there was scriptures considered as authoritative. And in the first century (B)CE, the Book of Daniel was one of these scriptures.
Regards
A.