Woah. You weren't kidding when you said you had some things to say.
I guess I'll start by saying that I agree. I've seen others talk similarly about Celestia and how she was practically neglected for much of the show as well as getting the short end of the stick in terms of characterization, and your post pretty much confirms that.
The show writers bungled Celestia, and I think the fandom just rolled with it. I also get the feeling that Luna is more universally agreed by the fandom to be cool while Celestia is just... all over the place, I guess. Luna's got a solid and dedicated fanbase because her character has a bunch of things going for it. Celestia meanwhile can only offer a few bits of character with a side of vague implications for her fanbase to latch on to.
On the topic of Daybreaker, I can see how she only exacerbated the problem of the fandom's skewed views about Celestia. It's a symptom of the lack of characterization for Celestia. I do also think that Daybreaker would have been very cool if we had something from Celestia to base Daybreaker's character on.
I doubt I'll be able to say much else about the Celestia situation since I think you've already pretty much said everything there is to say about it (except for the whole sibling rivalry thing which I hope you'll touch on at a later time), so I'll just get off-track and talk about headcanons now.
Anyway...
I've always felt like Celestia is a divine figure in the same way that some historical people like Alexander the Great or Constantine XI are immortalized as legends - their stories embellished with myths and folklore. For example, Constantine XI is a kind of King in the Mountain to the Greeks that would come out and save them in their hour of need. However, Celestia is also kind of an actual divine figure in the sense that she has the power to be able to move the sun and moon and is very magically capable in general.
I guess what I'm trying to say is that Celestia to me is in some sort of limbo between a god and just a particularly powerful pony.
I have this similar view with all the other powerful beings of MLP. Luna, Discord, Grogar... I view them with the mindset that there are no gods, only godly power. Of course, that doesn't mean that people won't praise them as gods or deities anyway. I think of Celestia as the romantic image of a ruler of legend spliced with a bit of the mysticism and power of a deity then spliced with a gentle and protective motherly figure and then spliced once more with a generally loving and caring princess of the people.
I realize that with the how unimportant the show made Celestia, this doesn't really make sense, but I'd like to think that in a world where the show actually cared for Celestia (as much as it did for Luna at least), that would be her portrayal.