[…] I think it’s rather unfair that in an episode where the reality-threatening danger was directly created by Luna, in an episode where Luna herself refuses to seek her sisters help or guidance, Celestia is still somehow the biggest criticism.
Well, you start with the fact that Celestia wasn’t in the episode, and work backwards. Luna doesn’t seek Celestia’s help or guidance. Why?
(It’s because up to this point, Celestia has been useless in a crisis and consistently made bad decisions, and because her relationship with Luna hadn’t been properly fleshed out.)
After how Celestia is characterized in Twilight’s Kingdom, including her in this episode would have been completely tone-deaf. Like, what would Celestia even say? “Just get Twilight to fix it and then never think about it again. Works for me!” This is an episode about taking responsibility, and Celestia lives in a world where you don’t have to do that. Her not being in this episode is yet another symptom of how she’s written.
Which brings me to:
Another problem was that Luna’s guilt and self-loathing seemed exaggerated, because if we look strictly at the events shown, Luna didn’t really do much harm, certainly not enough to warrant creating the tantabus to torture herself.
if we look strictly at the events shown
And there’s your problem. You have to look at what the events mean to the character. Luna and her sister were supposed to maintain harmony. Instead, Luna lost control. In a fit of anger, she attacked her own sister and destroyed the castle they built together. For 1,000 years, Equestria was down an alicorn, and her sister was forced to rule alone.
The reason that Luna is such a compelling character is that she takes responsibility for her actions. And her emotional journey is about the difference between constructive and non-constructive ways of doing that.
[…] everything Celestia does is put under such heavy scrutiny.
We hold good to higher standards. That’s what makes it “good”. And we expect the “Big Good” to be a voice of reason and know better.
I think Starlight’s backstory is a lesson in how “no reason” is better than “a bad reason”.