We don't know what Ivy knew that evening, or at what what point she knew it. We know that by morning she knows, and feels guilty as all get-out about it. As for the why being primarily to hurt Karen, she is more trying to hurt Derek in retaliation, and Karen the same tangentially, as she believes, mistakenly, that Karen was aware of Derek's cheating with Rebecca.
With regard to the ring, she kept it because Dev was treating her as if contact with Ivy (rather than his own choices and actions) had soiled him - with contempt and disdain, while demanding that she make every effort to find the ring for his true beloved, Perfect Princess Karen (which is in Ivy's possession only because of Dev's own stupidity and carelessness) for him.
I agree with you. I'm positive Ivy didn't steal that ring on purpose. I remember Dev leaving it in her hotel room by accident. Dev asked her to find it for him and Ivy just neglected to give it back to him in a timely fashion after he treated her like a whore afterwards. Even if Ivy did purposely sleep with Dev to hurt Karen, it is still completely implausible and unprofessional that Karen would have the power to fire Ivy from the musical for anything other than the actual job she was hired for. What makes it implausible from a writing standpoint is that someone with Karen's credentials would never have that much power over a Broadway production. Never. Living legends like Bernadette Peters or Patti Lupone, quite possibly, but it would still be the talk of Broadway. Saint Karen of Iowa, an unknown and unproven actress until Bombshell, having that much sway to fire an actress over a personal issue between them is unrealistic to that world and that's what makes this plotline ridiculous.
Derek is such a cartoon of a character. He's like some classic evil British villain in a old Hollywood production. I just laugh and laugh at him in every scene because he's not a real person to me. It's like the writers thought he should be the lovechild of Simon Cowell and Elton John.
Edited by Turkish, Feb 6, 2013 @ 12:00 PM.