I see no reason to assume that the Daughter of the Nine Moons is a person. Why could it not be Mat's spear? The doorway said that he had to marry the DotNM when he asked why he had to go to Rhuidean. The Maidens call themselves "married to the spear". Why can't Mat be married to the Ashanderei (spelling?). The thing is likely related to the Court of Nine Moons since it does have those ravens on the side. Besides all of this, it isn't Jordan's style to have a character assuming something like "The Daughter of the Nine Moons is a person whom I am going to marry" and having that character be right. Generally, his characters tend to make false conclusions. If an assumption is made, it's probably wrong.