MA, i'm aware of but not conversant with with hardiness zones in your land and would make this suggestion re the hardiness of queen charlotte. i think your information may be quoted from unrelated sources. as a violet, queen charlotte is a survivor with no reputation for tenderness. i think you may assume if you can grow other odorata types in your zone, you will have no trouble with queen charlotte. good luck!
rob...

I have reliably had Queen Charlotte come back in my yard for several years now. I am in zone 4, and although we get lots of snow, we also have wind blown and exposed frozen soil much of the time too (we live on a very windy hilltop). You should not have any trouble with them in zones 6/7.

Deanna