Ellebracht comments that "This native Latin word [Pontifex], taken from [pre-Christian] Roman cultic language, is used in the ORATIONS to mean bishop"; and, perhaps a trifle tartly, adds "The expression summus pontifex ... as a designation for the Pope is of later origin and does not fall within the scope of this work".
Exactly so. The phrase made its way into the euchology of the Roman Rite when, in 1943, a new Commune (Si diligis) was inserted into the Roman Missal for summis pontificibus. On the Festivals of a canonised pope, if he had previously possessed his own Mass formulae, he was allowed to keep them. But if, before 1943, he had been observed by the use of one of the Communia for bishops, he was now, so it was decreed, to be commemorated instead by the use of this new Pacellian Commune. Its Prayers described popes as Summi Pontifices, supreme or sovereign pontiffs.
I possess an altar Missal in which some conscientious person has inked in the dozens of changes needed in order to give effect to the new ordinance. And the new commune is pasted in at the back. (During the war, papal tinkerings with liturgy could not be imported from the great continental liturgical publishing houses; so this page has the imprimatur of Bishop Edward Myers, with the title Vicarius Generalis deleted and replaced by Vicarius Capitularis.)
This new commune also provided that, in Masses of summi pontifices, the praefatio of the Apostles should be used ... as if a Pope is another Apostle!!! Irvingite!!!!
This was the high tideline of Pacellian aggrandisement of the papal office. Soon (when??) this use of the Preface of the Apostles was withdrawn because of protests. And when the post-Conciliar Missal of 1970 was published, the phrase summus pontifex had disappeared from its communia. It had lasted just 27 years.
It is my view that, in this detail, the change made in the 1970s was desirable. It is one reason why my enthusiasm for the 1962 Missal is rather muted. A mere 27 years do not give a significant liturgical innovation auctoritas.
The impression given by the Pacellian dispensation was that a Pope is an additional tier of the Sacramental Ministries ... so that we would have Deacons, Presbyters, Bishops, and Popes. This unfortunate impression was reinforced when Joseph Ratzinger, after his Abdication, continued to wear a white soutane (a garment associated in the popular mind with the papacy) and to be referred to as Benedict XVI.
During the period when the Great Schism of the West was being resolved, papal claimants accepting demotion were, I think, often given one of the Suburbicarian bishoprics.
That still seems to me the rational thing to do ... liturgically ... with a former bishop of Rome.