Golly!
Even today when we can say we might have living eye witnesses to a situation one person might call another flirtatous while someone else there would say not at all. It's all rather subjective. And in this case how do you prove something from so long ago when you can't get two people to agree as to another "flirting" yesterday?
I'm going to take a long shot guess though using astrology.
GD Marie was a Cancer. If anyone understands or gives relevence to the signs, she physically portrays a Cancer quite well. That would be where her sweetness comes from. Cancer is emotional, tender and
protective.Now I could well imagine a young lady such as this who is cooped up and worried about her family. Being nice or friendly and hoping perhaps to soften the imprisonment would not seem unusual to me. I could imagine all of the IF using what people skills they had to help the situation.
Also her parents, I'm sure, would not have allowed anything less than befitting a Victorian Grand Duchess.