I never thought that about Lucy--it's been a while since I've read it, but I was always under the impression that a) she felt threatened by Edward's love for Elinor whether or not she thought either of them were going to act on it and b) she enjoyed being mean because she could. I don't know whether she realized that Elinor would stop pursuing Edward right away but I also don't think it makes that much difference in how she acts either way.