"The line between sexy and slutty is in knowing exactly how much to reveal."
I somewhat agree with this statement, but I don't think it's as cut and dried in terms of the rules you state. IMO, that line is different for each woman. Whether one is perceived sexy or slutty has to do more with a) how the piece of clothing fits one's body type (i.e. the same T-shirt that looks innocuous on an average woman might look differently on someone with exaggerated proportions) and b) how a woman projects herself (i.e. her body language, confidence, speech patterns, etc.)
I thought the dress was daring, yes, but appropriate given the setting and even when applying your 2 out of 3 rule. The front and back were cut low, but the slit is actually hidden in the folds of the gown (there's a copious amount of fabric in the lower half of her gown). You actually only see it because she raises her leg in the first panel.
I haven't read much of Huntress so it's not jarring for me to see her in this outfit (personally, I think she looks most beautiful here), but I understand if her fans might have a different opinion.