Found it, here's the writers' reply
about the question of Luna's whereabouts during the Royal wedding. They said more will be revealed in the future. And judging by Luna's attitude during the episode, Order's theory might be correct.
Biostar wrote: Where has that symbol showed up before? When I saw it I was thinking:
"Swirls and stars? Stars and swirls..."
"Starswirl the Bearded?"
But that's just me.
It's almost a 100% chance. He is implied to be one the greatest magic user in history, so I don't think there's a higher level than that in terms of magic.
Celestia and Luna are powerful, but their talent is not magic, and as shown in the S2 finale, it's not hard to get stronger than them.
About the villain, I can sort of accept him given how the episode played out, but shouldn't they have at least included his motive? I mean, Luna became bitter and turned into NMM because she was bitter that no one appreciate her, Discord did it because it was fun, and Chrysalis needs to feed her subjects, but what was King Sombra