I didn't know Debra Messing or John C. Reilly could sing. You clearly didn't see CHICAGO or the last Academy Awards Ceremony where Reilly happened to be one of the high points of a rather dull night in his song & dance number.
The second part of your comment was negligible. Messing is a fine comedienne and Miss Adelaide is right up her alley, her singing voice is perfect for the role (she loved making fun of her singing in WILL & GRACE but she sings much better than her character Grace did), she's extremely sexy--which is a must for the role in this particular revival (though she doesn't have the innate sexual persona of Jane Krakowski, one of the many reasons I'm sad she isn't transferring with the production), and the kind of comedy the role requires comes natural to Messing (from NED & STACEY to WILL & GRACE to THE WEDDING DATE you can see how she's an inspired choice to play Miss Adelaide).
Hathaway's singing voice is perfect for Sarah Brown and perhaps casting someone much younger than the rest of the cast (if the cast does end up being the Hathaway, Wilson, Messing, Reilly foursome) will add an interest subtext to the piece. It'll definitely explain Brown's naivette and further explosive behavior once the Havana scene happens, it'll create an interesting sexual dynamic between the older Sky (Wilson), and it'll definitely make Adelaide's plight all the more painful since she's so much older than young Sarah Brown. Let's just say it's gonna be a riot seeing Hathaway and Messing singing "Marry the Man Today."
God, I can't wait for this revival!
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"