What she had to gain: Possibly money, possibly she had a grudge based on personal reasons, possibly she has a political agenda, or maybe she is telling the truth...take your pick.
She's not stupid.
How do you know this?
If she doesn't have proof, she's an idiot for suing him because her career is over.
Perhaps she really is an idiot, perhaps she wanted money and only filed because O'Reilly filed first, perhaps she is what cBS would call an "unimpeachable source" (e.g. a complete nut job like Bill Burkett).
O'Reilly's in deep trouble on this one, boys.
Maybe. But I'm dubious.
Fair enough (you're being dubious), but I'm going to worry about you if I hear you've been sitting in fields waiting for the Great Pumpkin on Halloween.