I love this novel. I'm glad you're thinking about it so deeply.
Curley's wife is lonely and has few choices if she wishes for human contact, making her reliant on that men to circumvent her loneliness.
In 1937 Steinbeck adapted the novella for the stage. When referring to Curley's wife, he wrote:
"She is not highly sexed particularly but knows instinctively that if she is to be noticed at all, it will be because someone finds her sexually desirable. . . . Her craving for contact is immense.
His wife, Elaine said:
"I asked John once, 'Why didn't you name Curley's wife?' And he said, 'For one good reason. She's not a person, she's a symbol. She has no function, except to be a foil -- and a danger to Lennie."
The men themselves fear getting in trouble with Curley and are sick of him, ultimately making them biased against his wife and making their testimonies against her (such as when Curley gossips about her to George near the beginning of the novel) either null or so biased that it would be ill advised to take those assertions as truth.
Kind of. The only other women in the story are whores hanging around at the same 'cat house' where Curley found his wife. Every man on the farm (including Lennie) reduces her to a sexual object - which is why her name is ironic. The 1981 and 1992 film adaptations both follow Steinbeck's line and portray her as sexually aggressive or adventurous. If you like movies, Sherilyn Fenn plays up her vulnerability.
Curley's an interesting character. While he gains his authority from his father, Curley attributes the swampers' deference to his boxing prowess. If I recall correctly, Slim makes the observation to George (who warns Lennie) that Curley has started fights and beaten other 'big' men before and will go after Lennie with no other provocation.
Her advances with Lennie were more likely platonic that sexual. She looks down on him, but he's simply the easiest one for the characters to relate to, and they were in a position where they were unlikely to be interrupted.
While her interest is largely emotional (not purely physical), Curley's wife is interested in Lennie because he beat the shit out of her husband. He's also the only character who isn't smart enough to leave her alone. I think it is worth mentioning that Curley wears a vaseline lined glove to keep his hand soft for her. Curley also uses his hands to fight (he's a boxer). When Curley provokes him into a fight, Lennie breaks his hands.