Is it the general belief that Nach is unchanged since it was written? That the transmission of Nach is as reliable as that of Torah? What types of methods are in place to ensure its accuracy and authenticity? For example, a Torah is disqualified if even a single letter was added/deleted, must be entirely copied from another scroll, the scribe must be trained and learned, and other checks and balances. Does the writing down of Nach require the same?
"Is it the general belief that Nach is unchanged since it was written? "
Nach is not as strictly recorded as the Torah is. The DeadSea scrolls for example show many variants on Nach. And you will notice in Nach many "corrections" marked in the mikrot gdolot. The differences between various versions of Nach are not extreme though. I.e. there is no version that praises something, and another version that condems it. The average person reading through all the manuscripts of Nach, without a critical eye, likely would not even notice there are differences.