The Only way to remove what you are describing as a very deep key line, would be paint imo, using touch up, for me just doesn't cut it. Speak to people like Chips away etc to see if they can recommend a way without the costly exercise in having it professionally painted.
I'd not be happy with anything other than the way it was before! I do feel for you, sometimes people can be very evil and jealous!
I sincerely hope you get this sorted, it would kill me if it happened on mine.
G