http://www.scientificamerican.com/article/dawn-spacecraft-sees-spots-as-it-approaches-mysterious-ceres/

In the latest images, taken from 46,000 kilometers away and released on February 25, Dawn has sharpened its view of mysterious bright spots dotting Ceres’s crater-pocked surface, some of which were previously seen in the Hubble images. What used to appear as Ceres’s brightest blotch now appears to be two—a brighter, larger spot next to a smaller, dimmer one, both in the same crater. “Bright” is a relative term—all the bright spots are actually quite dark but still far brighter than the rest of Ceres, which is blacker than coal. No one knows what the bright spots are but guesses abound: Perhaps they are scars from recent impacts or minerals deposited by active geysers or water ice erupted by “cryovolcanoes”—or something even wilder. In 2014 the Herschel space telescope spied transient plumes of water vapor tentatively linked to the approximate locations of the white spots in Hubble images.
(Emphasis my own).

SciAm probably has it right, even though it had me fooled. It appears to be a combination of your geological ideas, and a result of the image processing done to stretch the contrast (it was done deliberately, because of the dark tones of both the spots and background). Its analogous to sunspots, which are white-hot, but appear black in contrast to the Sun’s even hotter and brighter photosphere.

Ceres is just visible to the naked eye at favorable oppositions, which is remarkable considering how small and dark it is, and its location past Mars’ orbit.

Still, I wouldn’t discount the Deathstar hypothesis just yet.