I think that Mary's callousness in itself is not that remarkable.. I think that particlarly at that time, people WERE inclined to calculate "how long so and so had to live" becuase to soem extent, it was through the deaths of people that estates were inherited or opportunities for promotion etc came up....So WIlliam hoped for opportunities that woudl probalby only coem through the deaths of fellow sailors etc.
I dont htink she ever cared two straws for Tom, and she is selfish and cool, and Edmund WOULD be a more responsible estate owner than Tom is ever likely to be..
But I do questin whether she woudl really talk about it so openly to Fanny. She must know that F is much more "proper" than herself...or if she doesn't she's stupid....