The Bible is not just a book about god, it is a book about the ethnical history, the legends and cultural developments of a certain culture in the Middle East over 3000 years ago. It is also a poetic book, an ethical manual, and a ritual descriptor. Whether the god it claims to exist exists or not is irrelevant to the criticism of the different parts of it. For example, we know that Genesis is bs, not because god does or does not exist, but because it is demostrably wrong. The slavery period on Egypt, though, is actually (more or less) correct. So are the descriptions of the two temples of Jerusalem (disregarding details such as ancient jews believing pi = 3, and being unable to use measuring tools), the rites, and the descriptions of what did those ancient jews believed: Cosmology, ethics, religion, spirituality. It doesnt matter if it is true, or not. Te important thing was that people believed it, and acted acordingly. It tells us a very interesting story.
And btw, atheists do not believe there is no god. Atheists do not believe there is a god. There's a difference. While "God exists" hass a dichotomical truth value, thats not the matter in question. "Do you believe god exists" and "Do you believe god does not exist" are different questions, and one can say No to both (Explicitally becoming an atheist on the first question, becoming an unnamed ideology on the second, and arguably admitting agnosticism on both).
Disregarding this extremelly stupid thing, lets get back to your questions, on a purelly atheistic viewpoint.
Was the Bible intended as a historical document, at all, or just a theological guide?
Both. As I said earlier, the distinction between factual history and legend is relativelly recent. People believed on the desertical flaming bush talking to Moses as they believed on the destruction of the Temple of Jerusalem. Both were history for them, only recently we can distinguish exagerated truth from legend.
if it was intended as accurate, is it literally true, or could some be merely metaphorical?
Hard to tell, but I think it was mostly intended to be transmitted as accurate. Most of the bible was transmitted via oral tradition, AFAIK, until the settlement of jews in Judea. The hyperbolic and parabolic style of writing could be blamed on thousands of years of oral tradition. However, some books, as the book of Ruth, are clearly fable-like, and dont seem to have oral tradition behind it. Ruth's book, specifically, could have been written as an attempt to de-demonize Caananites, which were a hated ethnicity to jews, just as the Samaritans on Jesus times. Aparently didnt work, though.
Could errors have snuck in from human copying, because God would not have impinged on the free will of those doing the writing?
Surprisingly, as the Dead Sea Scrolls prove, Jewish copysts were extremelly efficient. Development of checksum techniques for checking the accuracy of a writing made errors extremelly unlikelly, and, as a result, Torah's version of the bible barelly has changed in thousands of years of handwriting. We dont know how was the bible before it was written, though, so it is safe to assume that it suffered the same processes as oral legends did. On the accounts of Jesus on the New Testament, they were written at best 30+ years before Jesus alleged death, and by people who didnt meet Jesus at all. The most ancent texts we actually have are those by St. Paul, but it is this sinners opinion that Paul had an agenda, writing what suited his idea of Christianity, rather than accuratelly transmitting Jesus teachings, but thats just, like, my opinion, man.