I like the idea of Edith dancing (and I always liked Luthien). And really, considering some of the writings of Tolkien's contemporaries, I'm just glad that he created one Eowyn. Arwens and Shelobs are more typical of the era (personality-wise, of course, not spider-wise).
I think you're probably right that his audience wouldn't have been comfortable with a large number of strong-willed, non-conformist women characters. Arwen's the perfect courtly princess, and I can see how she would have been appealing in a world that was scarred by war, and changing much faster than Tolkien (and his peers) liked. And the spider-ladies certainly have a long (indirect) genealogy of ensnaring evil women, particularly in the Arthur mythos. I suppose Tolkien was writing in the "language" he knew best, in which will and fallability were male traits, and women who held them were often less than admirable. So bravo to him for making Eowyn so wonderful!
PS- Ungoliant- I like your name. Always liked those spider-ladies!