imo I don't think soveishu infertile. He and Navier didn't have a child together simply bcuz they don't sleep together all the time unlike Heinley never leave Navier alone. He insist sleep in the same room and bed every night, unless he is busy with work. But Sovieshu and Navier didn't even share the same castle. How are they gonna have a child like that. Semen can't teleport into vagina unfortunately
actually it was stated on the novel why sovieshu insisted that navier is infertile. they have history of why is it like that, that was why he was so sure that navier is infertile. i am not also sure that they didn't have sex even once. i am reading the novel (spoiler) alright, in navier's pov, she stated there that "alright, since i'm married to heinley, it's fine to do these things and that. how did i kissed sovieshu before? i don't even remember now." it was like that but not the exact thing. meaning, in her statement, she probably did it with sovieshu. i kinda believe that it's either both of them is infertile or literally both of them
Sovietshit kept on saying that the empress is infertile. what if just a year or two after the divorce, the empress is already pregnant with Heinley's child
XD