I am in agreeement that the KJV is sometimes inaccurate (more often, obscure and fatiguing) but I don't think it can be called plagiarized. It was deliberately intended to be a revision of the previous English Bible (the Bishops Bible but also the Geneva): in any event the translation of the same work twice is likely to show a LOT of duplication and similarities.
The OT of the KJV was supposed to be from the Hebrew (the Massoretic Text, which was then imperfectly available in printed editions); the LXX is the source of some OT quotations in the Greek NT but not all OT quotes, and the Disciples, being Jews, would know and use the Heebrew version.
As for "missing books" there are mentions of "lost books", such as the Book of Jasher, which simply don't exist anymore for any translation. The 1611 edition of the KJV (and recent deluxe editions) contain the Apocrypha, altho the KJV version was hardly an improvement over earlier translations and modern translations (e.g., RSV, NEB, TEV) are better.
Anybody who prefers the KJV is welcome to it, but I object to attempts to forbid reading modern versions.