The Great Synod was assembled at the end of the Babylonian Exile, at the beginning of the Second Temple period, about 450 B.C.
Why did they meet [at Yavneh] if it had already been sealed?
The meeting at Yavneh was a regrouping of the remnant at the time of the destruction of the Temple. They were under a severe calamity and the purpose of the meeting was to salvage what they could of the religious observance now that the Temple was destroyed. The rabbis at Yavneh anyway had no authority to make any changes since the chief rabbis of the time resided in Babylon.
I don't know what book you're quoting from, all I know is that this is not the Orthodox rabbinic scriptural history.
It is too long to post, but I think is a good explanation of where we take the septuagint from. I am sorry about the length. I look forward to your comments.