I have no objection to Kelly's being a blond. It's just interesting to me that so many people read her that way.
Frances McDormand is certainly the right type to play Marian. Maybe ten years ago; she's comfortably middle-aged now. In trying to think of someone not conventionally pretty who could do the role, I was able to come up with only two names -- Amy Madigan and Ellen Barkin...neither of whom fits my admittedly fuzzy picture of Marian. But they're both good actors and could handle the role with no trouble.
I'm embarrassed to say I don't know what Laurie Metcalf looks like.