That's actually not true. UA, as described by the theologians, is a function of the infallibility of the Ecclesia Credens and actually derives from it. So, it is in fact true that anything which the Church accepts universally as being "of faith" must necessarily be so.
I didn't say anything universally accepted
as de fide isn't infallibly true. I said
anything and
everything universally accepted isn't infallibly true. Not everything is taught as
de fide, or accepted as
de fide.
But your explanation of the
passive infallibility of the ecclesia credens failed to mention that it
"depends and is caused by [the] active infallibility" of the ecclesia docens. Because the teaching Church is infallible when it defines doctrine, the believing Church is infallible in believing when it gives the assent of faith to what has been infallibly taught.
But UPA is slightly different. Since the Church is not infallible in
electing, the universal acceptance of the one elected is not "
caused by active infallibility". UPA is
caused by God and it serves as an
infallible sign attesting to the truth.
Since the Church is not infallible in electing, there must be something
in addition to the election that renders the proposition infallible
quoad nos. If not, how could the Church
ever be sure it had a true Pope? What if, unbeknownst to all, he wasn't validly baptized? What if the previous pope, whom they thought was dead, was really alive somewhere?
If the Church can't be infallibly sure that the Pope is the Pope, how can it be infallibly sure a "Pope" who defined a dogma was really a Pope? Since the certitude of faith depends on certitude that the Pope is the Pope, there has to be an
infallible sign attesting to the truth. If not,
everything the Church has taught in the past would become uncertain. If the sign is not present in some cases the Pope remains doubtful, and therefore is c
onsidered to pope at all, even if is really is the Pope. But if an infallible sign was
never present,
possible doubt would always exist, and
every pope would be more or less doubtful. That's why there must be an infallible sign that removes all
possible doubt.
That infallible sign is the peaceful acceptance of the election, and it happens as soon as the Church learns about the election and doesn't contest it. From that point on, he's the Pope and we're stuck with him until death do us part. We've had UPA with every Pope for centuries, including our great and glorious Pope Francis, or as I call him, the Divine Chastisement sent by God to punish the wretched Catholics today (who deserve
far worse), and to see who will remain in the Church
and keep the faith through the divine chastisement.