I don't know if he's trans, but he likely felt dysphoria because the typical male image was so different from his self image. "If I'm not like all the other men, then am I a woman?" A lot of men likely feel something like this through their life. It's different from "I feel more comfortable when...
Super obvious twist given the ambiguity around the dead cat and Seiichi's mom somehow being fast enough to go from behind Seiichi to grabbing Shigeru as he falls. Seiichi was starting to show antisocial personality disorder by killing an animal and his mom was never to see Shigeru fall. I doubt...
Because Seiichi was almost killed by his mother in this area, this might be the manifestation/hallucination of the trauma caused by his mother. Shigeru was made into an outlet for Seiichi's mother's rage just like Seiichi was before and this could be a way for Seiichi to finally sever his...