This game has two endings depending on what choice did the player choose in the dialog at the balcony. Basically after dropping all Fragments in the Gate Field and waking up, going to the balcony, you will encounter Yuri Saigyouji, who reveals the true purpose of the Fragments and a bit of what had happened to Mari in the past that pushed her into trauma. Then she gave Mari two choices: Go with her to Heaven while still alive as a reward for her good deeds and piety, or stay behind on Earth. There could be some dilemmas to this situation:

If the player chooses to go with Yuri, then Mari cannot return to live on Earth as a human, due to if she ever go there, her soul will be completely purified and becoming a Celestial (not neccessarily the same class of Celestials as Tenshi from mainline Touhou, but rather refers to a person who is ascended to Heaven while still alive, according to this AU's headcanon (No, I don't mean to say that people who has ascended to Heaven alive such as Jesus and the Virgin Mary are Celestials in this AU; rather they should be classified as saints rather than Celestials)). People would think she is dead while being unaware of her ascension to Heaven, and the same situation that has occured to her during her life might occur to one of her best friends (very likely Reira Hayato). But she could enjoy the bliss of Heaven and is free from her depressing memories, living happily among the angels if she ever do so.

If the player chooses to stay on Earth, then Mari could reunite with her friends and family, and put her sadness aside. She now can enjoy the happiness of being with the ones she loved again, but life on Earth is not as blissful as life on Heaven, and there could be a chance that she could be pushed into the same kind of trauma again if something terrible happened to her. Also, there could be a chance that she might recall the memories that she have been trying to repress through some random event on Earth (even with simplistic events such as a school show-and-tell), and Mari could return back to a shut-in again due to she become depressive after recalling a painful experience, leaving her friends and family behind again despite that she has started to rekindling her relationships with them. In short, despite she has put her pain and sadness aside and reuniting with her friends and family, they are still there with her, and might disrupt her happy moments with her friends and pushed her into the same depressive situation if something forces her to tap into it again.

Below is a meme I created based on this thing (Also a spoiler):

One can see either ending as happy or sad depending on how they consider its consequences on either Mari or those around her. (Also a spoiler)

Okay. That's enough. I don't want to spoil you too much, since that might be ruining the whole game experience, as I said above. Anyway, I hope you guys enjoy it, but please do not take this as official information regarding the game, since the game is still in the works, and currently no playable builds have been released, so please take the information above as pure speculations about the game's ending.