While I agree that we should worry about Patrick being in Mame's hands...she should have some amount of warmth and eccentricity that would make you feel better about sending him there than to an orphanage...with Gaga I'd want him anywhere but there. (They did actually approach Gaga about Funny Girl, though.)
Murphy would make a better Vera.
The real problem with casting Mame, which Almira hit on a bit is you have to find someone who:
a) Is eccentric without being forced
b) Looks like someone who has had money forever
c) Is reasonably naive enough that the Upson's disdain for Jews would surprise her
d) Is warm and loving, remember she has to deliver "My Best Girl"
http://www.youtube.com/huskcharmer