However, I can't help myself. To say "The pre-Christian era Septuagint was completed, gradually, by about 150 BC. We just don't know with certainty what books were in that canon because only seven pre-Christian books are known to exist."
Completed gradually from what? Bits and pieces? Fragrements located in many different places?
If we don't know with certainty what books were in that "complete" canon how is it possible to know one complete canon ever existed in the beginning?
As Bill O'Reilly says "I am a simple man". This simple man is of the belief you make a compelling case for the lack of a single, complete, authentic Septuagint.
Are we possibly talking past each other?
The Septuagint has never been located so it is believing in a ghost . There is no proof of its existence
From Pauline Epistles, which were written circa AD 40-60 we have OT references which do not correspond to the Masoretic (aka "Hebrew" Bible or the targums), from more than just the Mosaic books and the two minor prohest, so there must have been material considered scriptural in Greek which included other parts of the Old Testament.
Other New Testament authors writing between AD 70-100 do the same thing. Philo of Alexandria, a Greek-speaking 1st century Jews speaks of the Septuagint, and so does the Jewish 1st century historian Josephus. Origen (latter half of the 2nd century) didn't doubt its existence, nor did any other Christian apologetic as far as I know.
So, there is ample evidence to conclude that some canon of Greek-language Jewish scriptures existed and were being used by Greekl-speaking Jewish diaspora. Exactly what that canon contained is not entirely certain, but obviously it contained some books not otherwise found in the Masoretic Text.