[Galadriel55]

I don't think that Elfhelm knew that Dernhelm was Eowyn, but he did know about Merry. I don't know how Eowyn convinced hi to keep quiet about Merry, but I'm quite sure that she did not reveal herself. I've thought of her commanding Elfhelm to keep silent, but then it says that all the troops seeed not to notice Merry. It would be a bit hard to keep hundreds of people fro gossip that would sooner or later reach Theoden's ears.
About Eowyn's feminism. It is not bad; she has it and had it all along. It was just shoved into her deepest corner and replaced by dreas of warriors. This was sharpened by the years that she spent looking after Theoden and feeling her helplesness and lack of power. In a way, she was overcoe and blinded by that dream. When she talked to Faramir, he woke the other nature in her. It is a mistake to call it feminism, because it is also present in Faramir and other people. It is more like the desire to preserve and create rather than destroy and kill. It is not the kitchen that Eowyn returns to, it's just a different for of freedom.
Eowyn is not THE woman, as Tolkien views them. Look at Arwen, Galadriel, Rosie, and Goldberry! They are totally different, by character, by their "role in the family" and their idea of what they could or should be.