The first Christians weren't Bible Christians

[Aractus]

My point is that the versions of the OT scriptures used by Christians from the first century to the 5th century changed dramatically. People who think they can trace the LXX further back than the Hexapla work under the assumption that the "proto LXX" bore substantial resemblance to the fifth column, and that is not supported by evidence. Paul seems to quote from the proto LXX, however it's important to remember that we can never know if Origen modified verses specifically to better agree with how they appear in his epistles. There are some scholars who are so sceptical of the LXX that they think no Greek language OT was ever quoted by the NT and put down any agreement with the LXX to Origen or others before him modifying the text of the LXX.

The Eastern Orthodox church still today teaches that the MT is corrupted and the LXX is not, and indeed it wasn't until the discovery of the DSS from 1946 that the Roman Catholic church was finally forced to reassess their belief that the LXX was more authentic than the MT. The theory ignores the customs of the Jews of the time, and attempts to re-write Jewish history! Here's an example of this invented history lesson at work - and again, here where they claim the DSS are fabricated by the Jews.

I believe that Paul and some of the other NT writers made use of a proto-LXX, I don't think you can say anything about whether this proto-LXX contained the Apocrypha and I don't think it was a complete volume of the OT. The reason why I don't think it was complete at this time is two fold: in the mid-second century you had several new translations of the OT into Greek all appearing at the same time, and textual criticism of the LXX tells us that different translators translated different books separately to each other, and that the translation quality is not consistent. We also didn't have codices until the first century, so prior to this it had to exist solely as a collection of scrolls.

And I will take not of this and find out if it is true

Well it is true, but I'll help you out by telling you exactly when this occurred.

The LXX as we know it is the fifth column of the Hexapla written by Origen c. 245AD. Origen modified it by completing sections missing from the Hebrew, comparing it to the other Greek versions, and for whatever other reasons now long forgotten. The sixth column of the Hexapla is the Theodotion translation of the OT. Theodotion probably made use of the LXX in his translation and it's not known what his original textual basis was. All the earliest manuscripts containing the LXX - eg Vaticanus, Sinaiticus etc get their Old Testament text from the Hexapla, none of them are free of the changes made by Origen (thus influenced by Theodotion and the other Greek versions), and no manuscript of the LXX copied from an older source is known to exist. The scribes who made these manuscripts generally followed the fifth column (that'd be the LXX), however in some sections they followed the sixth column instead, and that is exclusively true for the book of Daniel. Thus we know pretty much exactly when the LXX version was discarded - it was after 245AD and before 350AD, that's only a window of 100 years. We also know why - Jerome wrote that churches read the version of Daniel according to Theodotion and not the version in the LXX, and he was clearly familiar with both. Exactly how much influence Theodotion had over what we now have as the LXX is now unknown.

So to sum it up:
First century: Hebrew OT and proto-LXX
Second century: complete volume of proto-LXX plus other Greek translations of the OT
Third century: Hexapla - churches seem to have adopted the use of OT books from the LXX and they adopt to using Origen's revised version
Third-Fourth century: Churches read OT books from the LXX, except for Daniel which they read the Theodotion version instead. Scribes start following Theodotion in part when making OT manuscripts especially the book of Daniel.
End of Fourth century: Jerome translates the Vulgate, he follows the LXX according to Origen's version, and the book of Daniel according to Theodotion's version.