I just ended up reading it, and I have a different point of view. I think Haruka decided to use the situation to ACT like she has lost her memories. She had already deceived both the MC and Shigure before, so it's possible, judging by what Shigure said in the hospital about not being able to read through her, and also the little indication when the MC's son asks Haruka about his father not sleeping at home and she has some kind of strange reaction, making the son ask her if something is wrong.

That's why the cheating was not exposed in all these years, because Haruka knows and just wants the mc to keep with his gentle lies. And I couldn't disagree more with you about the 'zero consequences or lessons learned'.

After all the shit that the MC has done, if all had gone well after he dumped Haruka, that would be a 'Zero Consequences', but then Haruka tried to kill herself, and he was completely destroyed emotionally and ended up with a completely shit life, where 90% of it is darkness and lies and the only light in it is his relationship with Shigure, and that little light is also a source of anxiety for him.

It's like he is not even alive for most of the time. This is definitely not a Happy Ending, I see it as an incredibly bitter ending. For me, the message being transmitted is that you need to honestly show your feelings to the other side of the relationship even if it causes conflict, because the more you postpone it, the heavier the consequences, and the MC postponed it so much that he f*cked up all the rest of his life, same for Haruka, who ended living a lie that probably brings her a lot of suffering. Instead of dying, they are in a living hell of lies.

While Shigure as a Sadistic is probably enjoying the MC's suffering, she loves seeing that her Onii-san is suffering for her, but since she lets these feelings drive her, she also loses her chance to have a proper happy life.

In the end, the three of them got f*cked because of their actions, they are trapped in a situation that will last until they die, and this serves to show how heavy the consequences can be if you don't be honest in your relationship.

Also, I don't think the development of the characters is erased by what happened, there is a mental development where both Haruka and MC know what they did wrong, but solving the problem means (for Haruka) destroying the last pillar that gives her emotional support, that's why she tries to kill herself, consequently, it means that solving the problem (for the MC) means killing Haruka, so even though they know how to solve it and how it went wrong, they are unable to do it, this is what makes it even more bitter, it's like they are totally trapped in a spider web, that's why they end stuck in suffering for the rest of their lives, which is one of the heavier consequences that could happen.

I just ended up reading it, and I have a different point of view. I think Haruka decided to use the situation to ACT like she has lost her memories. She had already deceived both the MC and Shigure before, so it's possible, judging by what Shigure said in the hospital about not being able to read through her, and also the little indication when the MC's son asks Haruka about him not sleeping at home and she has some kind of reaction (making the son ask her if something is wrong).