This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Yuuta did not die in the fire.
There was a reason for the autopsy scene where the an investigator explicitly asks if the body is male or female. I don't think the author didn't know the differences between male and female skeletal structures. Rather, it was a clue that Yuuta is Not Quite Dead and someone else died in his place.