By the way, I'm not saying it was a mistake because Mitsuru "knew what would happen" or anything like that, when he went to the hotel with Kouichi. It's just always a bad idea to go "private location" for a closure talk, because things can just happen with lingering feelings... and in this case it ended up being a *disastrous* mistake because when Mitsuru did actually reject him, Kouichi went psycho rapist manipulator - which (unsurprisingly) Mitsuru didn't expect.
I think I freaking love you! I have always thought there was a way to explain the mistake Mitsuru made without victim blaming him. And you just made it so effortlessly clear! Yes, lingering feelings are always going to make your judgment less rational. In hindsight, yes, he probably would have known that, but, then again, when your judgment isn't rational that ALSO includes your judgment of your rationality not being rational, either.
I think Mitsuru's only (but in hindsight, major) mistake was agreeing to go somewhere private, alone, with Kouichi. It feels like there was always something unhealthily co-dependent about Mitsuru and Kouichi, he's always way too easily swayed by what Kouichi says. He tried really hard to get over Kouichi and he was doing a good job of looking properly at Shougo (although he should've just changed his phone number). Wanting closure by talking to Kouichi properly is normal and can be a good thing, when your ex isn't mentally unhinged like Kouichi...
I think that Mitsuru truly didn't WANT to have lingering desire for Kouichi, and didn't want to sleep with him. He wasn't planning on it and didn't want it. But once it was happening, it's almost impossible not to feel that relief of "Ah, he does still love/want me" about someone who broke your heart. Still it was non-consensual and I feel sorry for him because he definitely paid for his mistake many, many times over, which he did not deserve.
So yeah, the real mistake was agreeing to go "somewhere private" because they "couldn't talk about this in public". Get fucked, Kouichi. Either go to a restaurant with private rooms so it's still semi-public, or just fucking whisper. Sex with the ex is often almost impossible to avoid without iron self-control, if you didn't break up because you stopped having feelings for them.
Shougo is wonderful and he certainly didn't deserve to be hurt by any of this. I still hope he and Mitsuru can find a way to be together (and Mitsuru can learn to be stronger as a partner) because Mitsuru was at least always straightforward with Shougo and they WERE getting to the point where they could truly make each other happy.
I don't care what happens to Kouichi, at this point. I have no sympathy for him at all. I know we never saw his and Mitsuru's relationship when it was good, and we don't really know WHY he's such a spoiled nutcase mama's boy (if there's even a reason). But I doubt it's going to be a Hadakeru Kaibutsu situation, where I truly got to understand and emotionally connect to Kan-chan.