Though I adore Jan Maxwell, I feel like she isn't right for this vocally. I'm not intensely familiar with Mame, but she sings (and belts) quite a large amount of material in the show, correct? Though Maxwell isn't a bad singer, I don't know if her strengths are primarily vocal, at least not for the demands of this role.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.