Totally agree. I suppose it is unfair that Mary doesn't inherit. But it feels just as unfair that Edith and Sybil weren't going to. And that Rosamund didn't. And, while it is unfair that some people are never going to inherit because of their gender, it's much much more unfair to me that some people are going to have do really ridiculously hard work and graft - like Daisy, Mrs P, Mrs Hugest - and that some people, because of the accident of birth, can make one mistake and have their children taken away (Ethel.)
I don't feel this great tragedy about Mary's position, frankly. Not least because she will have known her WHOLE life that she wasn't going to inherit, this wasn't some massive surprise for her. For most of her life, she will have expected to have a younger brother so it's not like she always expected to marry the heir, she was brought up with the assumption that she'd be leaving Downton when she got married. And, I'm sorry, I also find laughable this idea that she is now working for the estate - as Cora would say, she's not exactly going down the mine. She still seems to have plenty of time to go to parties, sit around having her hair brushed and to be spiteful to her sister.
No, I don't think that's so clear. When she wanted to show her American grandmother "what Downton was about", did she show her the village? No, she threw a grand dinner.
What she's been doing on the estate has been about moving towards farming the land themselves (i.e. getting rid of tenant farmers) which benefits her and the family, not other people. I see absolutely no evidence that she's interested in the village as a whole. There are a couple of individuals who are beneath her like Anna who she'll be nice to but, when Anna disagrees with her (as we saw over the contraception), she has no problems ordering her to do things she's uncomfortable with.