The Codices are complete works. Textual criticism is based on complete works being compared to existing fragments. Based on that, the Vaticanus and Sinaiticus have been used since their discovery as the more correct version.
would agree knowing the care that the Jews took to preserve their Scripture. They wouldn't have stood for some insertion
Well, the Septuagint (fragments) and the Dead Sea Scrolls were written by the Jews as well, and they don't agree with the fragmetns that correspond to the Masoretic Text.
Besides, the New Testament uses almost exclusively LXX as its source.
Something being a complete work as opposed to a fragment doesn't say if it reflects the original. It is something that the scholars (several of whom were antagonistic to the idea of biblical inerrancy) have surmised.
Regardless, it is impossible to say for certainty that one thing was an addition and one was a subtraction. We don't have the originals so, you believe what you believe and I'll believe what I believe. Luke makes it a non-issue. She had a firstborn Son. His name was Jesus.