I feel that this is a difficult subject to tackle. Although I think the best way to understand something is to experience it (in this case, understanding queerness by being queer), ultimately there can't be a completely accurate way to go about a story unless it has been been experienced first hand. But what does that say about any other movie or television plot line? A movie made by a queer director with queer actors could be entirely inaccurate in a queer sense if none of the above have ever experienced whatever story is being presented.
I don't think that in "Heavenly Creatures" having a straight, male director detracted from its queerness. It may be true in many portrayals of queerness that the view is perverted and skewed to fit the idea of what is generally acknowledged as queer (say, in just about any movie or television using queerness for ratings or expanding the fan base), but Peter Jackson's film told a story of an unorthodox "friendship" between two girls, nothing else.
As for the movie itself, I do believe it is queer, even with statements saying that the friendship was completely non-sexual. It shows an obsessive and extremely intimate relationship between two girls that is not what is considered "normal." And not normal=queer, right? With or without the sex, the friendship was romantic in a sense, just in the imaginary world that Parker and Hulme shared. As for other aspects of a queer horror film, it pretty much fit the bill with over-dramatization of emotion, the constant shrieking, and fantastical element.