I'm going to work backwards some:
Minsc, discussing Gale's relationship with Mystra:
"Gale reminds me of the vremyonni of my homeland. The man-mages of Rashemen. While the girl-folk go on to rule as wychlaran, Weave-touched boys were hidden away. Trained to work their craft in silence and secrecy. It is an old custom, not well-observed. In truth I thought it born of caution, after some catastrophe wrought by wizardly men-folk of old. Now I wonder if it was not done to hide them from Mystra, and the snares she sets for young and prideful boys, hm?"
Minsc is not a reliable reporter. Ever. At all.
(from memory) " do you think Minsc could forget all of that?! . . . You are right! Minsc has forgotten it all "
But he's not the only one confused, no Rahsmeni man knows the reason behind this practice.
Rashmen is a Matriarchy and, like Barbieland, it has it's injustices. But the reason behind this practice is not to hide boys from Mystra, it's to prevent the restoration of the demonic patriarchy that used to rule the Rashmeni lands. Before there was a Rashamen there was another country in a continual state of war of the demon-worshiping patriarchy of Narfel. The men who ruled proto-rashmen used forbidden magic to take down narfel and in doing so destroyed both themselves the patriarchs of Nar. The Wyclaran matriarch keep these secrets - including Imaskari secrets hateful to the gods - but keep them out of the hands of men because they fear that the demonic spirits that remain in Rashemen will whisper to them about the glories of a restored demonic patriarchy.
This doesn't justify the injustices of Rashemen but does explain it . . .
Long story short, Minsc is wrong.
The timeline is also problematic. How old is Gale? Late 30s? Early 40s? From appearances alone he looks mid 40s. While his prodigal talents might have captured Mystra's interests before then there is no reason to believe that she started the romantic relationship at an earlier age than she started her relationships with other chosen - mid 20s in Elminster's case. This even bolstered by the evidence that he had lovers before Mystra.
As to the identification with Arabella it may indeed be the that Gale identifies with her because he wishes that the mistake he made as an adult could be treated as a youthful indiscretion. But the 40 year Gale was not literally a child. Given the tremendous differences in power between Gale and Mystra he might feel like a child when dealing with her but, again, he was not actually a child.
(although in terms of chronological age Mystra-Midnight is younger than Jaheria. And probably younger than both Halsin and Dame Aylin)
Now if you think than any age / power gap relationship is so problematic as to be criminal and are willing to apply this to similar figures like Isobel and Aylin you would be on solid ground. If you think that Isobel's early faith in the Moonmaiden was a form of grooming that set her up for an inherently abusive relationship with the Selune's daughter you are standing on solid ground. But by suggesting that Gale was not at least in his 20s when the romantic relationship started you are on building a castle on a weak foundation.
And you are placing a very great a weight on a rotten beam.
Fiction, be it speculative media or realistic fiction, is a safer way to examine these subjects than nonfiction where real and specific human beings are attached. Because it is a very heavy subject, such narratives won't be what everyone is comfortable engaging--but it is absolutely within grounds for creators to tell those stories regardless. Speculative fiction allows audiences to connect with human beings who may have experienced very different lives and circumstances than them, but who share common humanity. What aspects of humanity are shown depend on the storyteller and the story itself, but it isn't inherently trivialized or framed with less seriousness than examination of the topic in a different time period or part of the world. Additionally, you could argue Astarion being subjected to enslavement and rape for two hundred years is an overly strong analogue to apply to a 'silly story'--but it is canon. This is the story being told in Baldur's Gate 3. Touching upon child abuse in a story built to examine abuse across cast members would not be out of place thematically or in terms of severity. Whether the abuse is examined effectively and respectfully might be subject to examination, but it does have a place in storytelling and evidence suggests it was incorporated deliberately here.
Lovely. Ah! I wish I could write as well as you. But beautiful words are not always true and true words are not always beautiful. You are right to say that fiction can be a safe way to explore heavy subjects BUT, as you have acknowledged, you also need to ask if your treatment of the matter trivializes it.
And, my articulate interlocutor, I fear you you have done just that. You have trivialized the issue of child sexual assault in by applying to the silly story of Gale and Mystra.
This is indeed a silly story in a way that Astarian's is not. The child abuse analogue is a reaaaaal stretch. Beautifully written, well researched and well argued. But it's a tin foil hat theory that is shoehorned into an abuse story in a way Astarian's is not. ( said fondly, ** the rabbit re adjusts their own foil hat to ensure ears are covered ** )
Astarian's story is, root and branch, a story of abuse. One of the first sincere things he says to Tav is "I was a slave . . ." And, of course, we have more than a century and half of vampire stories that deal with this weighty topic. Even if Astarian's story was a weak one (it's not) it would under-girded by decades of vampire stories that addressed these same issues.
Do you know the comic book the Killing Joke? Even if you haven't read it you probably know it because it has influenced every "dark" version of Batman since the 90s. It's easy to find nerds who celebrate it and credit it with establishing graphic novels as a form of literature.
What's less well known is that the author of that book has decided that the critics were correct and he has denounced his own book. He decided that a comic book could not deal with an issue as weighty as sexual assault and that his use of it in the story was exploitative. And I have enormous respect for his decision to do so!
Now if one of the most influential and celebrated products of nerd culture failed to carry the weight of a topic as heavy as adult sexual assault do you think a fan theory about a video game is suited to the task of allowing us to explore the issue of child sexual assault? I don't and I do think it trivializes a very important issue.
I also want to express to you, as someone who has been involved with media for popular nerd properties in both a consumer and company capacity--child abuse is very much examined in works like this.
But this isn't that is it? It's not an examination, it's a fan theory. And, frankly, it's a slightly offensive one.