First off, assuming the Female Changeling is the same individual as the First Humanoid woman who died 4 billion years ago makes no more sense than assuming that Brunt and Shran are the same person. Sometimes one actor plays more than one role. No sense reading more into it than that.
Second, it makes no sense to equate the Preservers with the First Humanoids, because the Preservers are a modern group, not an ancient one. Think about it. They transplanted Native Americans to another planet because their mission was to preserve endangered cultures. Well, Native Americans weren't endangered until Europeans colonized their continent in the 17th century! Not to mention that the Navajo, one of the cultures Spock claimed to be represented by Miramanee's people, didn't exist as a distinct culture until the 17th century (they were sort of a hybrid of the Pueblo and another culture). So the one and only instance we have of Preserver activity dates from only 3-400 years ago, our time. There's absolutely no evidence that they were around in more ancient times. (We don't even know if they are a distinct species or civilization, for that matter. They could've just been an organization like Greenpeace or the Audubon Society.)
And it makes no sense to equate a four-
billion-year-old civilization with any currently existing species or group, because the time differential is just too monumentally long. Species tend to last a few million years, civilizations only a few thousand. Remember, the First Humanoids seeded the primordial soup of worlds that had single-celled life at most, if even that. It was billions of years after that before any multicellular life emerged on Earth. We're talking a yawning, incomprehensibly vast chasm of time between then and now. Given the way evolution works in the Trek universe, the First Humanoids would've probably long since evolved into an incorporeal form like the Q or Organians.
Just looking a reason they took the shape they did. Maybe the first solids they encountered looked like that.
The Changelings we saw in DS9 took the form they used in order to mimic Odo's appearance, which in turn was his approximation of the Bajoran form. We have no evidence that any Changeling took that form before they met Odo. Remember that Laas, who independently adopted a humanoid form before meeting Odo, picked a totally different appearance than the one Odo used, basing it on the people of Varala, where he learned to adopt a humanoid form.
