You make a number of interesting points, Nicholas. I would strongly disagree with your theory that the Hebrew Bible used by 4th Century Jews had been translated back into Hebrew from the Septuagint. The evidence seems to show that there were several text families of the Hebrew biblical books none of which was a retranslation back from the Greek. What does seem clear is that the Septuagint represents a group of Greek translations made from different (and sometimes earlier) Hebrew texts that the texts that 4th Century Jews were using.
The criticism of St. Jerome as a single translator has some merit. But then again, we know the kind of translations that you would get from a committee. Despite all of its faults, the Vulgate was a momentous achievement for its day and a definite improvement over the Vetus Latina texts that preceded it. And despite many criticisms by the early Humanists, the Vulgate text was adequate for its time and did not contain any doctrinal errors. Sadly many anti-Catholic controversialists have pretended that the Greek texts were clearer than that Vulgate when in reality, they had much that was hard to translate and virtually every vernacular translation even in modern times rather simplistically gives us only one translation when in fact there are several plausible ones.
This is why the idea of a perspicuous Bible is laughable. What we have is a powerful literary resource that conveys the inspired word of God but in a form that is not always clear. When we admit this, we can more honestly study the scriptures and stand humbly before the mysteries overlie its messages.