As for Edith I do think Tom is going to end up being key to how the baby situation plays out. He doesn't have the same blinders about Edith the others have and I feel like there's been a lot of him figuring out how much he cares about the family and they care about him this season.
It feels like they are setting up for him to do something big for the family (I don't necessarily think marrying Edith)
Heck no one knows his family beyond his brother so he could have a 'niece/nephew' that he takes in when a relative 'dies' allowing Edith to keep the baby close.
Or even more possibly he could claim that the child is his and not Edith's a minor scandal but far less then if Edith publicly has a child. He could then bring the child home when the mother dies while Edith is abroad and there you have it Edith gets to help raise her child (as much as she would've as an upper class parent)and no big scandal. This would also serve to permanently cement Tom staying at Downton he'd wouldn't do that and then complicate matters by leaving.
So I think they are definitely building to something with Edith and Tom but not necessarily a marriage.
I'm not sure I see Tom and Edith building to anything, if only because there have been very few scenes between the two of them this season and I feel like there would be if they were going in that direction.
From the previews, it looked like Edith might confide in her aunt and the aunt strikes me as someone who would know someone who could "take care of it." So I'm wondering if they're working towards some kind of abortion story line.
As for Mary, I don't think they'll commit her to any guy in this season. I think they'll throw out a lot of possible guys, but they won't last unless one has particular chemistry with Mary. Then they can bring him back next season.
I still like Tony Gillingham for Mary. I know he's engaged now but wouldn't it make for a better storyline if he had to break his engagement to be with Mary? Really though, I thought they had great chemistry.