Alan, good luck with your book! It's a great topic. I can't contribute much, other than the following subjective opinions.
1. Traditionally the planets, (and now with asteroids,) the planetary meanings took their rulerships from mythology, not from their astronomical properties.
For examples Mars (Ares) was the god of war. Mercury rules liars and thieves today because the young god Mercury (Hermes) was a trickster. Venus (Aphrodite) was the goddess of beauty even though we know today that the planet Venus is unbelievably hot and inhospitable to life.
Why the imprint of ancient mythology should be the case today when we no longer believe in the pagan gods, I can't say. However, I am one of those people who thinks that astrology works through the human mind in connection with the horoscope as a means of graphic communication; not in what's physically Up There in the Sky. And then I am also learning of some quantum physicists' view of the participatory universe.
http://discovermagazine.com/2002/jun/featuniverse Basically the theory is that the way light is emitted from heavenly bodies or photons in the lab depends upon the observations.
2. Sedna's mythology and even her name seem to have a lot of different versions, which isn't surprising, given that Inuit (Eskimo) people ranged from Greenland to the Bering Sea, and had limited contact with the more distant bands. In some version's Sedna's Unsuitable Suitor is a fulmar (sea bird,) a sculpin (fish,) or a faithful dog, or she is simply a girl who angers her parents. A common theme, however, is that she angers her father while they are in a boat at sea, and he casts her overboard. When she clings to the side of the boat, he cuts off her fingers-- which become the creatures of the sea. She became the goddess of the sea-bottom, and is often angry.
When hunting is bad, the village shaman metaphorically travels to visit Sedna at the sea-bottom, and combs her hair (which she cannot do, being maimed,) and then she will release the marine mammals to the hunters. In astrological terms, we might call Sedna a malefic. Like Pluto and the Babylonian Erishkegal and Greek Persephone, Sedna is a goddess of the dead; although the location is at the bottom of the ocean.
Common themes in the different Sedna myths seem to be maiming, abandonment by the father, betrayal, and a life in a dark, mysterious place; that become the source of enormous power and creativity.
I have Sedna in Aries in the 8th house in a partile sextile to my Aquarian sun. The mythology of Sedna resonates deeply with me. My first massive crush occurred in high school, and never worked out. I thought he liked me, and we were friends, but no romantic overtures were forthcoming. From my father's point of view, this was the Unsuitable Suitor, and he was perfectly beastly to this young man. At that time, I was a piano student, and just poured my heart out on the keyboard, with hours of practice. (fingers:creative force) I have always found solace in nature, and as a high school student I found solace by a large lake by our home. (water connection.) My hair has always been unruly (Uranus square ascendant) but I've very seldom found a good match with a hairdresser-- unlike Sedna's shaman who combs her hair.
There is a lot of shamanic mythology embedded in the later more "civilized" mythology, having to do with the descent to the mysterious lands of darkness, transformation, and return. (Cf. Joseph Campbell.) The shaman's journey may be one of greeting the death goddess with loving kindness instead of fear.
3. I'm not a big fan of astrological octaves in general: in a horoscope interpretation, they do exactly what?? Even so, I don't see Ceres mythologically as a good match. Persephone (Proserpina), maybe, but then she's an asteroid, not a dwarf planet. Several years ago, I spent a lot of time researching dwarf planet Ceres as possible co-ruler of Virgo, and I looked at a lot of horoscopes, but gave it up. I still don't understand astrological Ceres. She has to be more than a junior moon, no? I think we need to understand Ceres a whole lot better before we octavize it.
4. Back in the 70s, I was privileged to have two field seasons in the Canadian Arctic. It is a vast, incredible place where life is hard for its native inhabitants. I hope you can get up there to see one of the Arctic oceans first-hand, if you've not done so already. You've written here about darkness: cf. the dark Arctic winters, when hunters go out on the sea ice. There's some geography in Sedna's mythology, I believe.
But good luck with your project, Alan. Just my 2c.