The more I read about the history and theology of the magisterium, the more I realize that it is super complex. The idea of the Magisterium itself is simple and most people readily understand it, including both of you. But, if you study the history of it, you'll realize that theologians over the last few centuries, (especially after V1) have changed, added and refined many of the definitions/labels of the terms.
The reason for the confusion and disagreement over the meaning of ordinary Magisterium, and ordinary and universal Magisterium, is due to several factors, one of which is that the phrases have never been defined, and another is because, as you said, the meaning of ordinary magisterium has not remained consistent. Here is a brief history of the terminology beginning with the theologian who coined the terms ordinary magisterium and extraordinary magisterium.
The first theologian to use the terms ordinary and extraordinary Magisterium was Joseph Kleutgen, in Die Theologie der Vorzeit Vertheidigst (1853). The purpose of the book was to counter the error of “dogmatic minimalism” that arose at the end of the previous century. The adherents of this position maintained that only solemnly defined dogmas constituted the ‘rule of faith’ and required the assent of divine and Catholic faith, while all other doctrines remained in the realm of free opinion. Kleutgen rightly rejected this. He argued that, in addition to defined doctrines, those doctrines that are clearly and unmistakably taught in Scripture also required the assent of Faith, and therefore could not be rejected without the note of heresy.
Some of the examples he cited were the doctrine that Christ was transfigured on the Mount of Tabor, that there are eight beatitudes (not five), and that Christians must love their neighbor as themselves. He said revealed truths such as these, which are so clearly taught in Scripture and universally believed by all Christians, form part of the rule of faith, even without having to be solemnly defined.
To avoid the accusation of Protestant “private interpretation,” Kleutgen added that the understanding of the revealed truth in question would have to be that which was universally held and taught by the ecclesia docens (Teaching Church). He readily admitted that he did not intend to include all teachings of Scripture in the rule of faith, but only those that have been constantly taught as revealed truths by the entire body of Bishops.
He included as part of the ordinary [and universal] Magisterium, the unanimous consensus of the Fathers and the “theologians” (i.e., the theological schools from the 12th to the 18th centuries), but noted that these are not teachers, but rather ‘witnesses’ of revealed truth – that is, they testify to the fact that a doctrine is taught universally as revealed.
He dedicated a section of his book to explaining why the shortest path to determine if a doctrine has been proposed infallibly by the ordinary Magisterium is to consulting the theologians. He said if these schools were unanimous in believing that a doctrine is a revealed truth, this is clearest sign that an undefined doctrine constitutes part of the rule of faith, and requires the assent of divine and Catholic faith.
It is important to note that since Catholics must hold that the faithful believe on the authority of God revealing and the authority of the Church teaching (not on the authority of the ‘witnesses’ witnessing), he again emphasized that the motive for belief was not only God revealing (in Scripture) but the ecclesia docens teaching. That is why he emphasized the role of the Pope and Bishops dispersed throughout the world, since these alone are the legitimate “teachers”.
This last point has been entirely misunderstood by some today, especially the sedevacantists, who interpret the statement “when the Pope and Bishops throughout the world teach the same doctrine, they are infallible” in a way that Kleutgen did not intend. What Kluetgen meant is that a doctrine that has been constantly and universally believed as a revealed truth, and is therefore presently taught as a reveled truth by the Pope and Bishops dispersed throughout the world, the doctrine is to be considered infallibly proposed, and hence demands the assent of divine and Catholic faith. The error of many today is to separate the diachronic universality of the doctrine taught (the what), with the synchronic universality of the legitimate teachers teaching (the who). The result of this error is the belief that the ecclesia docens has violated the infallibility of the ordinary and universal Magisterium, which has led to the monstrous heresy that the entire Body of bishops lacks authority - a heresy rooted in a misunderstanding of the infallibility of the ordinary and universal Magisterium, as understood by Pius IX and Vatican I.
Someone in this thread stated that everything taught by the ordinary and universal Magisterium is necessarily infallible. That is absolutely correct, provided the meaning of the ordinary and universal Magisterium is that which was intended by Kluetgen., since according to him only the revealed truths that had been constantly and universally taught as revealed truths constituted teachings of the ordinary and universal Magisterium.
It should be kept in mind that the purpose of Kluetgen’s book was to counter the error of dogmatic minimalism, by defending the position that some truths (not all, but some), which had not been solemnly defined (by the extraordinary Magisterium), nevertheless were part of the rule of faith, and therefore must be accepted with the assent of divine and Catholic Faith (i.e., and not open to free opinion).
It should also be noted that Kluetgen used the phrase ordinary Magisterium, not ordinary and universal Magisterium, but as I will explain below, what he meant by the former is identical to what Vatican I meant by the latter.
Tuas Libentur: The first papal document to use the term ordinary Magisterium is Pius IX’s Tuas Libentur (1863), which was addressed to the Archbishop of Munish-Freising. What occasioned the letter was a Congress that had been organized by Johann Joseph Dollinger, who would later leave the Church due to his rejection of the dogma of papal infallibility. Since it was strongly suspected that the Congress would be defending the error of “dogmatic minimalism,” Pius IX sent a letter to counter it, and he did so by using the phrase ‘ordinary Magisterium” - the phrase Kleutgen coined in his book that was published 10 years earlier for the purpose of countering the same error. Joseph Kluetgen coined a term that was adopted by the Magisterium!
De Filius: The phrase ordinary and universal Magisterium was first used in De Filius (First Vatican Council). It is found in the section directed against “dogmatic minimalism,” and Kluetgen was invited to the Council and appointed as one of the main drafters of the document. The initial phrase used was Kluetgen's original term ‘ordinary Magisterium’ and almost every Bishop opposed it. They did so on the basis that it was a novel term that could easily be misunderstood, and noted that even amongst themselves there was disagreement over what this novel phrase meant. After further clarification the Bishop approved the term, but added the word “universal” for the purpose of clarifying that it did not refer to the Pope alone, but to the entire body of Bishops dispersed throughout the world.
In later years, the term “ordinary Magisterium’ began to take on a meaning of its own (distinct from the ‘ordinary and universal Magisterium’ and from the ‘ordinary Magisterium’ as Kleutgen original intended it), and was even used in this different sense by Pius XII. We now hear about the Pope’s ‘ordinary Magisterium’ but the context in which it is used proves that it is not limited to teachings that have been infallibly proposed (which is how Kluetgen original intended it). Instead, the phrase ‘ordinary Magisterium’, as it is normally understood today, refers to the teaching of a Pope that is promulgated authoritatively (which is not the same as being proposed infallibly). That is why the assent that is said to be owed to teachings of the Pope’s ‘ordinary Magisterium’ is that of ‘religious assent’ - which is a conditional degree of assent that is owed to non-infallible teachings - not the assent of divine and Catholic Faith, which is owned to infallibly proposed doctrines that constitute the rule of faith.
You can see why there is so much confusion over the meaning of these terms. Not only are the terms new and undefined, but the meaning has evolved over time.
But the take away from this is that the meaning of the phrase ‘ordinary Magisterium,’ as used in Tuas Lientur, and ‘ordinary and universal Magiterium’ as found in De Filius, must be understood according to Kleutgen’s original meaning, since that is how it was correctly understood at the time these documents were written.
This is a brief history of the phrase ordinary and universal Magisterium, and how it was initially understood. For anyone interested in this topic, I would highly recommend the book, ‘On the Ordinary and Extraordinary Magisterium from Joseph Kleutgen to the Second Vatican Council,’ by Dr. John Joy, which explains all of this in great detail and documents everything.