This right here explains everything i want to say and better
Revealing the rape would the quickest way for Seth to prove the mitigating circumstance that he needs to prove in be able to save himself from getting his soul destroyed, as it proves why it's plausible why Seth would have been so desperately scared of Osiris that he'd even resort to commiting the very crime he might get destroyed for (which is the crushing of human souls just to create the very only weapon that might have worked against an undead like Osiris). To prove why he was so desperate to protect himself from the dead Osiris, enough to commit such a serious crime just to create that one weapon, Seth would have to reveal the rape in to show the real extent of Osiris' harassment and obsession with him, which would in order prove why he had a legitimate reason for becoming desperate and thinking that an already dead Osiris would keep being such a serious threat to him, even from the afterlife...
I totally agree with that, and I totally want to see Osiris burn in a firey pile of ash, but Seth doesn't work like that. Seth is prideful, and it's very clear that this was very traumatic for him. Having his rape on blast to all of the Egyptians gods just to absolved him from his sins is far from what Seth wants. Seth probably deep down knows that if he were to come forward about what happened to him, he'd wouldn't be completely to blame. But he doesn't care. He was used by his brother, discarded by his wife, and his son not only isn't his own, but was almost kid because of him; all because Osiris was so obsessed with Seth that he was willing to do anything to make a baby that was his and Seth's only, that way he could bind them by "Seth's standards for marriage". Ultimately, if Seth were to come clean, it'd be like admitting to a moment of weakness, where Seth was desperate, drugged, and clearly not in the right state of mind to make rational decisions. As a prideful war god, the last thing he wants is to be thought as is sensitive and weak. He wanted to show that he has control of his situation and thus he acted out. Honestly, Seth is so loving and emotional, but he also has so many flaws that this shows how complex of a character he is, and how well the character writing is in Ennead.
I forgot to mention that not to mention, coming clean would mean that he'd have to experience the whole thing over again a third time. Seth remember the event all over again when Isis confronted him that one time. Seth probably doesn't want to experience that all over again, as sad as it is to say.
Honestly, the fact that you can see the literal fear in Seth's eyes when Ra mentions the truama that Seth was put through to drive him to so many unthinkable things is really heart breaking and telling. Seth doesn't want to relive the truama of being r@ped by Osiris, and probably views the whole experience as humiliating, degrading, and a moment of pure weakness. Yes, he'd be absolved of SOME of the blame, but it'd bring up such a traumatic moment for him again that I doubt that he wants to relive again. Seth is prideful, and isn't the type to admit when he's weak. I feel so bad for Seth, and as much as I want the truth to come to light so that dirtbag Osiris can eat shit in hell, it's clear the last thing Seth wants to come to life is well, his r@pe. Not to mention you can really tell that Ra really only sees this whole debacle as something amusing and that's so unbelievable shitty, but I think that's so interesting at the same time?? Like aahhh this is such a great story...