Is is telling that your theology requires that fixed and unchanging doctrine begins only once the Church Age arrived.
the establishment of the New Covenant marked the Church Age arrival, so the rules of the New Covenant were set into place at its inception and are unchangeable as long as the New Covenant is in effect.
So the rules are irrevocable until they are revoked? Not that I believe in Mormonism in the least, but by what standard do you judge that the "translation" of the "golden plates" was not a valid marker for the end of the Church Age and the start of ..., ..., of the Saint Age I'll call it for the lack of any better term.