Pendulum has some pretty good songs in general, but that has to be one of my favorites. Also, the idea of Seishirou being old enough to be Kamui's dad made me choke on my apple juice. I hope you are happy.
The thought might have crossed his mind sometime after Saya died. I think before he was too happily derping about with Kamui and Kotori to notice anything. Once she died, Fuuma had to grow up a lot, since Kotori needed someone to take care of her and his bff was gone. In retrospect some of the interactions between his parents might have seemed a bit forced. And Kyogo not ever saying anything about Kamui and Tohru after her death is pretty strange too.
Does Kamui have any clue that Saya had the hots for his mom? Or that his mom might have had a bit of a thing for Saya too?