I think the idea of her being so deluded in that way is very interesting! I personally find the theory a bit flawed in regards to how he would handle the situation (im just gonna call him bf for simplicity here)
I think in the theory that she is actually dating him, the idea that he ended his own life is easy to believe because in abusive relationships like that, especially ones where you care deeply for the person even if they are abusing you, your logic is very skewed. He saw no way out of this relationship because he did love mahiru, but she was too much, and he couldnt even break up because I assume the relationship was at least a bit manipulative even if she was unaware.

But in this theory, he doesn’t know Mahiru, or at maximum he is acquainted with her. He would have no problem with taking her to court or filing for a restraining order if she did something like that, because to him she is basically a stranger. I dont think he would end his life because a stranger broke into his house, even if she had been stalking him, because there are easier solutions to the problem than that.
I hope that makes sense, that’s just what I think! The theory can work better if they were just friends too, but I still think there would be precautions he would take even then