You know, I could have written this post a year or two ago :) Like you, I really don't like princesses: the idea that being pretty is the ideal, that a man will save you from a dilemma, and that all bad situations can be cured (and by a knight in shining armour, not by your own doing). Furthermore, I hate the tiny waistlines and huge busts, the frilly dresses, and the overdose of pink.
However, my daughter really loves princesses, much to my dismay. Like you we exposed her to PBS, Noggin, and other characters. We listen to Laurie Berkner, do sports (gymnastics and swimming), do lots of puzzles and reading, and have avoided princesses like the plague. Guess what, despite our attempts at avoidance, she found them on her own (no grandparents needed!). She doesn't watch the movies but she loves dress up, talking to her 3.5 year old friends about them, and plans to be Cinderella for Halloween. The more I bucked the system, the more interested she became in them. Since giving in, she still likes them but has moved past the "obsessed" stage.
My compromise was to accept the inevitable, allowing her to be what she enjoys, but to shield her from the old fashioned stories with ridiculous morals/endings. Maybe this is what you want to talk to your parents about. There is nothing wrong with them making her feel like queen for a day every now and then, encouraging her imagination with dress up and pretend, and doing girly tea parties and the like. Start with the positive (as in "I like how you love to make her feel and these kinds of things are okay") and then move onto the things you'd like them to avoid. Also mention that you might seem to them like a paranoid new parent but that you appreciate their support and help. Just so you know, you will get the ole head shaking "there was nothing wrong with it when you were a kid" reaction. Don't get into a power struggle, just keep positive and continue to calmly ask for their help with this self esteem piece. Reiterate that both you and your husband really want this for your child and maybe even throw them a bone with "maybe some day we'll look back at this and think it's a silly cause but for now it's what we really want and need." You probably won't change your stance but all parents like to feel like they know more than their kids and challenging them makes them feel like you think you're a better parent than they were. You don't think that so let them know it's not a judgement call on them, just your own personal feeling.
You are so right with not wanting to spread the nonsense and hoping to avoid teaching your daughter the lies we were fed, but you can't fight city hall. Pick a department instead and fight that!
Good luck to you, I feel your pain!!
L. :)