You wrote:
“There’s no objective evidence that Ambrose thought about the post-medieval IC theory at all.”
What’s your evidence that the Immaculate Conception was “post-medieval”? If Pope Sixtus IV was defending it in the 15th century, then it is clearly not “post-medieval”.
“I do not suffer from the acute RC propensity to read their modern theology back in into the ancient authors.”
No, you suffer from something much worse. You suffer from teh acute and peculiar Protestant tendency to imagine Protestant doctrines are biblical and were embraced in the early Church. We, on the other hand, are the early Church and always will be. Your johnny-come-lately sect will always be a johnny-come-lately sect.
The 15th century is on the cusp. Given the lack of support for the doctrine during the bulk of the medieval period, I stand by my claim.
It is a doctrine not taught by the apostles nor by their (truly) catholic successors.