I hadn't noticed that Henry made sure the coast was clear then either.

He joked to Mrs. Grant and Mary early that he liked Julia best and that he liked Maria because she was engaged. Mrs. Grant laughed it off and said Mansfield will cure you. She had no clue what he was up to then.

He tells Mary his pointed plan about Fanny, knowing that the rest of family would not approve. I think he knows what he is doing is wrong.

I begin to think as you say that Mary didn't think this type of behavior was wrong. Maybe for Mary this type of game playing was par for the course.

That would explain why she spoke so openly to Fanny about the rehearsers, and to Mrs. Grant when she said “I would not give much for Mr. Rushworth’s chance if Henry stept in before the articles were signed.”
Mrs. Grant was alarmed when Mary said this.
“If you have such a suspicion, something must be done; and as soon as the play is all over, we will talk to him seriously and make him know his own mind; and if he means nothing, we will send him off, though he is Henry, for a time.”

Mrs Grant has no clue this is all a game for him. Mary does.

Mary doesn't think Fanny is up to this kind of game.

Maybe it's not just games either. Mary already said she thinks marriage is a take-in where people are not honest with each other.
She seems to think interpersonal relationships are about power struggles evidenced in her reaction to finding out Edmund is to take orders at the end of ch.23.
She was very angry with him. She had thought her influence more. She had begun to think of him;...It was plain that he could have no serious views, no true attachment, by fixing himself in a situation which he must know she would never stoop to. ... If he could so command his affections, hers should do her no harm.
To me, she has a very inappropriate level of anger, all because he is going to do exactly what he assured her he was going to all along!

Maybe I'm reading to much into all of this. She seems to have a very cynical view of people, marriage, relationships...

She seems to have compassion for Fanny, and cares about Edmund. I'm not always sure what to make of Mary. hmmm