For Murphy, I haven't really seen anyone who I think could play her convincingly. But from various pictures and clips I've seen on the internet, I have a few ideas. I just haven't seen any of them in tv shows or movies, so I can't judge the acting talent.
-Taylor Schilling. I haven't seen her act, but do a Google image search. She just looks like Murphy. I can't think of any concrete reason.
-Kristen Bell. She's 5'1'' and blond, and from what i've heard, she's a decent actress. Has anyone here seen Veronica Mars, because I'd like some outside verification on this.
-Katee Sackhoff. She's already been suggested, but I think she might be able to do the role. Only seen a few clips of her on Battlestar, but she was decent in that.
Also, I nominate Hellen Mirren to play one of the Mothers. No reason, other than Hellen Mirren being awesome, and having a body that is way too good for someone her age. Hell, she could probably play Mab, if some CGI were used to make her look somewhat inhuman and ethereal. Only problem is that there's no opportunity for Mab to use a machine gun. And if RED taught me anything, it's that Hellen Mirren with a Machine Gun can make everything better.