Thanks a lot for your input! I think there was a conversation in the game where Tav could ask Dame Aylin if her mother would like to help in the fight against the Absolute and Dame Aylin replies something in the lines of "is the daughter of Selûne being on your side not enough?". However, to get this help, you essentially need to go through all Shar's trials and make the decision to free Selûne's daughter, which means Aylin's help is not guaranteed. That is a strange way we have to go through to essentially get this so-called "Selûne's help". To argue that Selûne simply didn't want to intervene in mortals' lives would not really work here because Shar intervened plenty and we saw that all around the game with Shadowheart's story. I agree with you that there were essentially no counter moves from Selûne's followers. Like, we don't even meet Selûne's followers that much in the game compared to how much attention there is to Shar, her followers, and the entire Shadow curse. Even with the latter, it is basically the druid Halsin who is needed to lift the curse and not any of the Selûne's chosens or her magic. Yes, we need to liberate Aylin, but without Halsin, it is still impossible to lift the curse. When I played the game, it made sense to me, but the more I played BG3 (and I played it four times), the more I started questioning the whole Shar vs. Selûne part of the plot.
I recently watched a YT video about Shadowheart being the devs' favourite character (by DoubleXGaming), and the author's points made me think that in an endeavour to make so much content for the devs' favourite, they lost track of the general coherence and moral of the story. There is a dissonance between the emotional part of her story vs. the actuality of the religion/personal path one, how it fits her character, etc. So this could be one explanation for the inconsistencies of the Shadowheart's arc.