After a lot of hypotheses, several visits to the library, and the adoption of a lion (he’s actually quite nice), I have come upon a set of conclusions. I shall present my reasoning first, to ensure that my statements are fully understood.
It is well known that Celestia is, at least, somewhat older than Luna. With Discord’s help (I don’t dare apply quotation marks), I was able to roughly learn the exact difference in age, as well as a range of years in which they both may have been born, and a general location (allowing for weekly geographical changes at the time). Additionally, even Discord’s vague knowledge of naming conventions in the region helped tremendously, and sending some letters allowed for confirmation of the names of various great-great-great-etc.-grandparents. With this information, it was possible to create a sort of heat map of the possible name combinations for the two sisters.
Based on my research, it is most likely that the filly who would become Celestia was born in the region that is now Ponyville and Fillydelphia. Cross-referencing with the time and place of her sister’s birth (which is up next), it is most likely that she was born in a year in which that community made use of purely descriptive names. This would track with various ancestors that were present in the area, such as Minty, Rosedust, and Ember. Seeing as she survived long enough to overthrow Discord, it is likely that she had a dull name compared to some similar examples (poor Cotton Candy), and that is one basis for my theory. My only concern is that this name may be too unflattering for revelation, though better than if it was based on her rumored hair color. Not going to stop me, of course, as I am in too deep now.
As for the filly who is now Luna, finding her name was somewhat more involved. According to Discord, she was a very young adult at the time of his banishment, which refined my search somewhat. I am certain that the Princess’ parents stayed in what was now to become the Ponyville/Vanhoover region, in the time between their births. Within that time period, there are many naming conventions that do not appear to connect well to the younger Princess. However, after several days of consideration, I turned my thinking around: why had Luna chosen to name herself as such?
At that point, I noticed a connection. One of the most ill-fated name schemes to come up so consistently, and with some finagling, the perfect match. In one year, dozens of fillies had been born with just about the same name, give or take a few dozen letters. One used variation on the name stuck out as fairly generic, possibly even the basis for the others. And by axing just one syllable, a consonant, and a vowel or two, the name could be converted almost perfectly to that of the Princess of the Night. With this, and a check to ensure that Celestia’s name matched the timeline, I was certain of my thesis.