For starters, I'm a rabid Luna fan myself. But I like to see what others have to say(finally).
I've seen some video where people have taken Luna's symptoms into consideration and concluded she is indeed depressed. That's a fair point. What she did was self destructive.
You're right, your ban didn't seem fair. I still trust and love Luna, so after witnessing that myself firsthand, I might not have a problem with her as Princess(then again I have poor judgement myself).
I still think blame lies on Celestia and Twilight for not talking this out with her. Both were more than capable, I guess they had better things to do like eat cake and read Daring Do.