Brian Schweitzer is no sure win for the Dems. He has his own baggage. Daines will win regardless of the nominee. The Dems are more concerned about trying to win back the House seat that they haven’t won in 20 years.
My guess, based on nothing other than my experience of Dem thinking, is that they'll replace Walsh with Keenan: it's a sop to the abortion-feminists, it gives her a chance to build her political bones (a la Abortion Barbie), and it prepares MT for the idea of Hillary! on the big ticket in 2016. Since they've lost the Senate seat anyway, might as well use the election to build on their ideological future.