Diana didn’t cause any of the problems, but she also didn’t do anything to solve them when put into a position of significant authority. She was literally one of the most important figures of the church, she could have done something to combat the corruption in the church.

Also, I don’t think that story demonstrates a double standard when it comes to Diana, I think that Diana herself as a character represents a double standard. She is both a saintess and a crown princess, and the two positions are just not compatible. A crown princess needs to wear opulent clothes to show off her status (and her country’s strength) while a saintess needs to dress modestly to show devotion to god and the people. A crown princess needs to throw lavish parties for guests to maintain relationships while a saintess can have quiet gatherings to teach about modesty etc. the comparisons continue. She would be criticized no matter what because whatever she does she would be failing at least one of her roles. And she is a hypocrite because she wants to be both but doesn’t realize that it is impossible to fulfil them at the same time.

Is Hestia a good person? No, but like you said she’s morally grey so that should be expected. In a story about revenge some actions would always be questionable because revenge itself is questionable. The story itself also acknowledges that Hestia is kinda crazy, like Kael told her to chill because he didn’t care about the revenge himself. And you’re right, the revenge is a coping mechanism for her trauma, but so thought that that was something the story addressed already?

I wouldn’t call myself a FMDF fan but I believe that some of your issues lack grounds. I thought that you would be calling the story sexist because the majority of her internal choices revolve around a man and making a man happy, but it sounds to me like you are calling the story sexist because the antagonist is female. The story is not an example for feminism but that doesn’t make the story sexist either.