Well it might be because the Moon is actually closer to Azeroth? She could actually be something like a Sun and uses the Moon as something to reflect or amplify her magic/power through it? Would explain the whole Eclipse business that popped up in the cinematic and why Balance Druids can use there new New Moon, Half Moon and Full Moon spell.
As for her name sake, we call her Elune but that might be a name we've given her? If you put timeline into play we know that safely Night Elves acknowledged her 10,000 years ago. Elune came across as something simpler to help with understanding thus that easy assumption became common knowledge and Elune stuck with it?
I reckon Elune is Azeroth's Guardian, though the Titans themselves have not really involved Elune openly to say she is part of them. We only can judge based on that Tear of Elune pillar of creation that there is some connection really.
As for being a prime Light candidate - it's honestly hard to say, I don't really say she is as high ranking as that. She can probably use both Light and Arcane magic in her arsenal as much as Sargeras can use Arcane and Fel really. But they've still kept both Elune and Naaru as enigmatic entities. But at the same time, thrown something in that has such a colossal backlash that it's a bit weird to throw that curveball into the mix to begin with.
She has a sentience to at least mate with Malorne to create Cenarius, Elune is real in that regard, we then can assume that she is humanoid like Titans because Malorne being a stag wouldn't be able to procreate a child with that humanity and stag mixture. Overall, it still throws more questions than ever before and that's before the mic drop of Elune being able to harness enough ability to make a Naaru.
Like I said what's weird about it all is the fact that Moonkin being Elune's champions and Balance spec has references to Elune while other's do not. Even their spells have Solar and Lunar qualities though you can forsake as just gameplay whimsy, it doesn't make sense to include it still if the direction is going another way either. It would be like not removing Metamorphosis from Demonology Warlocks while letting Demon Hunters have it too, yet Demonology is stepping away from that intended fusion and gifting it to Demon Hunters.
They made the effort to keep it and added to it even, by giving the Artifact to Balance as Scythe of Elune. Relating it to things like the Wolf tribulations too.
The sun hasn't been really acknowledged so I'm not sure that will suddenly appear out of nowhere. If it was then they would relate it to the Naaru of some kind since they are from the Chronicles - beings of Pure Light and they haven't. I mean they could but throughout the game I've never really come across the whole "Sun gives us day" or anything Sun at all.
I so follow your thought of things being built up but I reckon Elune is being built up to be something of importance to combat the Void, if the Void is as terrifying as it is... then what that Orc said on Broken Shore is true... We will have a huge fight that includes Azeroth, PC's, Sargeras, Titans, Elune and maybe more to come to face the Void Lords. With all those allies it will truly establish what a real threat the Void Lords actually are because the whole parts of the universe is trying to combat the Void.
Also as for Alleria, that was out of the blue. She's been stuck off Azeroth forever and I don't think High Elves will come into game or they would have with Quel'Thalas by now. And... I don't think she will be a sun worshipper... She will be the opposite actually. And that's not trying to spoil 7.3 for you. If you do then:
Alleria becomes one with the Void, she gets a Void form and is scorned by Xe'ra for doing even if it saved that Naaru, so yeah you won't be getting your Sun worship or deity that way sorry.