You don’t want to get into the universal consent argument, because it highlights just how crazy your homemade theory is:
No, I am presciding from it here in order to keep the considerations distinct; otherwise the two issues can get conflated. That is important when logically analyzing issues. You start be determining the principles in isolation and only LATER apply them to various scenarios.
Your constant puerile outburst are not helpful to the conversation.
So the question is, ASSUMING for now "uncertainty" regarding the identity of the Pope such as what transpired during the Great Western Schism (whether you believe that to be the case or not), WOULD IT BE sinful for someone who adhered to one pope to assist at a Mass "una cuм" one of the other guys. That helps illustrate the moral principles involved.