Caring who is the true pope is not equivalent to rejecting the other papal claimaints as being not a part of the Church.
Find me something in a pre-Vatican II book that you think supports you, and I will read it.
If the other claimants are part of the Church, you wouldn't raise a flag of holy war against them.
If the other claimants are part of the Church, you wouldn't move and risk your entire livelihood just so you could attend a Mass in a diocese that didn't pray for them.
It's a matter of fact that the claimants excommunicated each other, and the people clearly recognised those excommunications with their actions. And even if they hadn't, it's not the laity who decides who is and isn't in the Church.