So...much...comment.
Perhaps the city has a reason for being named Everfree? I dunno. I just thought the familiar name would help readers associate the fic with the setting better. From an anthropological (okay, bad word, but the closest I have) point of view, your idea is much more likely. But I want readers to remember exactly where this is at all times.
Celestia is currently the same size that non-evil Luna was. Luna is maybe a tiny bit smaller, but the difference is much less drastic. And, yes, Celestia is purely pink-maned. I should include that earlier.
When you use magic, and you're a freaking goddess, you can make it work the way you want it to, not the way logic says it would. That's my story and I'm stickin' to it. Okay, really, the walls are enchanted, but you gotta use the enchantment on purpose. Back when it was a military castle, that would be a REALLY REALLY useful enchantment. Now it's just kinda nice.
Morning Glory isn't supposed to be a sex spy. Sex is just part of her repertoire (and her best method for castle entry). She's a social spy, and does all that entails. I'll try to give her less sexy social work in the future. "Backroom politics" is really the area of her work. Dirty? Yes. Sex? Not always. Lots of backstabbing and blackmailing and double-dealing and stuff.
The fic is going to be a lot about Celestia (more her than any one OC pony), but the OC ponies are all important for a reason that will become obvious in...I'm gonna say 3 chapters. It might be 4, but I'll try to keep it to 3 so people don't have to spend as much time wondering "why am I reading this?". I promise, it'll make sense!
I'm sorry that you hate Shimmerdust, but the way you described her is indeed how she is currently intended to be portrayed. Total freaking space-case. Also, her affectionate word choice when speaking of her family is intentional. And, yes, other ponies will treat her out-of-touchness as annoying, because to most ponies, it really can be wearying (especially when they're trying to do something important). I imagine she will get shaken by somepony during this story. Maybe even more than once. Celestia help me, though, I have too much fun writing her to even think about marginalizing her. As for Twilight Sage's romantic future...you'll have to wait and see.