Lad,
I'll try this somewhat differently. I'm not arguing against your rejection of the UA theory. Your argument would be a knockout if you showed the theory to be false on its own terms, or in application to the facts. For example, you'd show the theory false in application to a Conciliar pope if we had a sufficient number of ordinaries (can you think of even one, though?) who rejected a Conciliar pope: you'd have shown there to be no acceptance of the pope on UA's own terms. Checkmate. You haven't done that, that's all. So the UA enthusiasts would reject your view and it'd be a standoff.
Btw, I think your argument from cuм Ex is strong. I'd go with Paul IV over all those theologians any day.
So basically you're saying you'd prefer he took the theory as true and show it doesn't apply rather than show it is a priori impossible because of cuм ex and historical examples.
Obviously the latter is a stronger refutation.