"Linguistically, you are right. kercharitomene indicates that Mary had, previous to the Annunciation, had some sort of grace which had preserved her up until that point so that she could be the Theokotos."
You guys are doing fine on the Greek verb tense...but where do you get the "which had preserved her" part. That's not in the sense of the Greek word and putting it in there could be argued as demonstrating a denial of the free will of the Theotokos and that has truly astonishing implications for the Incarnation and veneration of the Most Holy Theotokos among other things.
I would agree, if I didn't think that preserving grace and free will were compatible.
I'm basing this on the reasonable inferences that 1. regardless of whether she was sinless or not, Mary's status was by a grace not her own, and 2. Scattered throughout the old Testamnet (e.g. Ps. 73) is the concept of a preserving grace.