I don't get how anyone believed Eve in the first place...
At least the way Anne Baxter plays her, she's so obviously phony in that cloying breathy way from the get-go. They're all supposed to be veteran theatre people and it's only Birdie who doesn't completely fall for it?
I still thought it was a great movie, but it required a lot of suspension of disbelief, more than you'd expect for a script riding on that plot point. I couldn't help but cringe or roll my eyes every time Eve said anything.
And I don't understand how Anne Baxter was nominated for an Oscar, unless she was supposed to seem that obviously phony for some reason? But it would've worked even better if she was more convincing, so that doesn't make any sense (and I know the Oscars don't mean anything, anyway).
On top of that, we never see Eve acting on stage so you just have to take the script's word for it that anyone would even want to see her act. She may be scheming, but she's just boring fake overly sincere scheming with zero charisma to me. And the idea that she could replace the lively spirit that Bette Davis has as Margo in anything just seems totally preposterous.