I think the gun seems a bit more like a reaction of hatred rather than nerves when unprovoked. What would be better (in my opinion) would be if Mary was startled just ever so slightly but gave a more realistic knee jerk reaction. You know like an instinctive rabbit punch to the throat or something. If you want to push realism a bit further (which I am all for), maybe she could rabbit punch the woman to the ground and then perhaps swing round and draw a gun on the guy from behind the desk. All so it looks like instinct, but that of an SAS war veteren rather than a business woman. Perhaps then she could, gently apologise with the woman still twitching for air as she leaves.
Just what I'd do.
Leading up to the end was good, the characters were developed quickly and efficiently. The concept generally is good, perhaps just look at how exactly the ending is executed.