Tbh the most irrational part of this story was them not giving a fuck about the will and doing the DNA test anyway. But well, DNA tests are expensive and the musician guy seems like the kind of person who'd feel sorry for the little girl's messy life and take care of her anyway because she'd probably end up in the system if neither of them were her father(which is a possibility like the mother said). He really is a big softie.
The seme just has a lot of traumas because of his childhood. Honestly, in this chapter, he acted like someone with anxiety. I mean, I suffer from it and I get caught up in my head in the exact same way. You can't stop being pessimistic and overthinking all the time. And obviously you hide those negative thoughts, except for when you're near your limit, like he did with his father.