While it’s true that some denominations have slightly different canons, they all still have the same set of core books. As for the other books, you need to see if it’s consistant with the others. There’s some that are fine, there’s some (ie the gospel of thomas) that are partly fine but also have some serious crap , and there’s some (ie the gospel of judas) that are full of crap.
How come that plenty of its meaning got lost across plenty of languages if there was divine help? How come that an English Bible and a Russian Orthodox one may say substantially different things, merely due to translation divergences that piled on during centuries?
This why new translations should always be translated from the original languages. Yes, sometimes there was no access to the original-language versions, in those cases good on 'em for doing the best they could with what they had.
why doesn’t God correct the record to make sure the hundreds of millions of Christians through the world who are following wrong ones aren’t led astray?
Not everybody is held to the same standard. If you do something wrong and you don’t know any better, it doesn’t count as a sin until you know. And being a Christian isn’t about what you do, it’s about who you know.