Catholics say it occurred at her conception, not at her birth. Her conception is past to Luke, as is her birth, as is her lunch the day before.
It is absolutely correct that Luke does not tell you exactly when Mary was completely graced as a completed and past act. However, since there is no question that original sin is present in all other humans (since Adam and Eve) from conception, it is at least perfectly reasonable and consistent with the scriptures to suppose that that past date, when Mary was perfectly and completely graced, was in fact at the moment of her conception.
There is a further argument here that doesn't involve /kecharitomene/. It's the argument from the Law. Jesus bound himself to keep the Law perfectly, including the commandment, "Honor thy father and thy mother". Yet, uniquely, he himself created his own mother. How can you reconcile creating his mother in a state of sin and enmity with God, while simultaneously "honoring" her perfectly in keeping with the demands of the Law?
If you could have created your own mother, would you have created her as an enemy of God and a slave of Satan? Would that have been "honoring" her? I don't think so!
You're basing the remainder of your argument on what ifs and doesn't it seem which is a very dangerous line of thinking.
Again, if this were the exception we should have record of it from one of the writers of the NT to clearly address this. That we don't is the key.
Then why didn't God create all of us without sin and save thew world a whole lot of suffering?
After all, Jesus says that those who believe in Him are His mother and sisters and brothers.
So why is it so important in the scheme of things that Mary be considered immaculately conceived and perpetually virgin?
What does it accomplish?
You'll need a LOT more straw!