You have never, in your entire life, changed your mind about anything or known anyone else who has? ... I don't think I can address that.\

Ok true Shinji changing his mind seems possible but i mean the point at which he chose to leave in your story leaves him without a reason to change it there's no guilt or pressure on his shoulders. In the series he became connected to the others and that is what brought him back. If he had the strength of character to defy his father and leave before the fighting began he clearly wouldn't see his father as a reason to return so what or who could have meant enough to bring him back? (I speak from the experience of BEING Shinji at one point in my life)