While some of the best Mames may have been English, that does not mean it is a must. Donna Murphy would be a fabulous Mame. If you saw her in "Wonderful Town," then you will recall the character of "Tracy" from the vignettes. In that one vignette, Donna shows that she can have that "life is a banquet" attitude. She also has the comic timing needed for Mame Dennis. As for being sophisticated, all you have to do is look at Donna Murphy in an interview, and she is sophisticated. An actress as phenomenal as Donna Murphy could easily bring that to the character. If you think about the 11 o' clock number, "If He Walked Into My Life," then you realize that Mame also has to have a warmth, and a softness about her. Which Donna also showed throughout "Wonderful Town." Donna Murphy has shown that she is a versatile actress, and frankly, I think she could play anything, except possibly the blonde ditzes that Kristin Chenoweth plays.
I disagree about Chenoweth as Gooch. K.C. has played the same roles over and over again. From Sally in "Charlie Brown" to Glinda in "Wicked" they are all the same. Gooch is not a ditz. She is naive but not a blonde.
The one thing I do agree with you on is Christine Baranski as Vera.
"Life beats down and crushes the soul and art reminds you that you have one."
-Stella Adler