My favorite HC for Mairon ever is that he was not created by Eru.
Eru did not directly intervene against Melkor but did so twice against Mairon, because Melkor is his child and Mairon isn’t.
Consider this:
Mairon is absolutely perfectly made and attuned for / to Melkor. He is his perfect counterpart, embodies exactly what Melkor needs at any point in time, to the point where he suspiciously represents the flame imperishable that Melkor so covets. They are obviously made for one other. And though one might interpret it as Eru not wanting Melkor to be alone through his suffering - I counterpropose:
When Melkor desperately searched and screamed for the flame imperishable (his mother if you will) in the void - she heard him. She heard the pain and agony of his suffering and knew what suffering awaited him. She disagreed with Eru on the matter of the fate of their oldest and mightiest son, her favorite son, who got the most of her in him. Eru damned him to his existence, I don’t think he would have cared about giving Melkor a companion. I don’t believe Eru thought his actions through fully at all.
But though limited in her contact to her children, the flame imperishable heard Melkor. And she answered.
And so another spirit was made - solely out of her will, solely out of her fire and music, another Ainu that she embedded a part of her fire in so a part of her could follow her sons call and not let him be alone. A mother always listens to her child’s screams and tries to give them what they need.
And so Mairon came to be, with the flame sitting deep in his chest and showing through his eyes, and when the flame sent him down to Arda, the Valar found him first. They didn’t know what to do with him, but they recognized a spirit of creation and so sent him to Aulë. His power unnerved the Valar, and they just decided to let him serve as a Maia, since he, as someone whom they didn’t see during the music, couldn’t possibly be a Vala. Aulë was at his mightiest when the flame imperishable lived in his halls. Mairon only left when Aulë fed too much on his spirit (cough, the dwarves. Ever wondered how they were even alive before Eru knew?)
It’s why Mairon felt so lost and with no purpose for so long. The music isn’t his purpose. Melkor is.
And so that’s why Eru intervened against him multiple times. He doesn’t have a plan for Mairon. He cannot foresee his actions since he did not make him. He can’t guarantee that Mairon will lose in the end, because Mairon won’t. He comes damn close to winning over and over and over again and he just doesn’t seem to ever stop, ever stop burning with the fire he was made from. So Eru just has to work overtime to contain the angry fire of his ex-wife‘s only child and prevent him from going against the music that he is not subject to. Truly the only creature in the world with free will.
This also opens so many possibilities - like an Angbang baby.
This adds so much more to future silvergifting. Just imagine the look on Tyelpë‘s face when he realizes the very flame of creation favours him.
