Actually, I'm not quite sure currently as the ensemble has been shuffling, but Rak certainly has never been a Muriel understudy...I mean I love her, but wow I can't see her as Muriel. She's one of Jolene understudies. The 3 ladies that have understudied her are Rachel de Benect, Laura, and Paula Leggat Chase. Julie Connor might have once been listed as her understudy, but I don't think she went on.
And as much as I adore these women, they really aren't physically/age appropriate for the character, only makes the fact that they understudy more amazing(there's aren't really any slightly oldish women in the ensemble), so I couldn't see them taking over the role. Besides Laura in particular deserves to originate a role on Broadway.
In terms of replacements...can Christine sing? I could see her...I mean Joanna's style is so unique it's hard to imagine anyone else, though I'm sure others could do an ample job.
Hmm, Andrea Martin comes to my mind...she certainly is funny.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli