Was William of Ockham the first sedevacantist?

It's frustrating when an asker of a question then goes on to provide what he or she considers to be the correct answer. Perhaps that is why nobody has bothered to give an answer until now (2 years and 4 months later). After all, what is there left to say when you provide fulsome quotes that appear to confirm your conclusion, namely, "This is sedevacantism because sedevacantism doesn't deny the papacy (as Protestants do)."

Based on the information you impart, I would agree with you. However, I would just point out that in the past, many a Catholic has claimed acceptance of the papacy when suspected of some error in that regard, to save themselves from severe punishment, in this world, not the next. I would also add a few interesting points about William of Ockham, some which partially challenge your account of this man.

He was a medieval English philosopher and Franciscan monk (1285-1349). Basically, one of his philosophical claims was that you shouldn’t make more assumptions than the minimum needed. It can be called the principle or law of parsimony, but this has a long history, predating William - e.g. Aristotle (384-322 BC) with a claim of the superiority, other things being equal, of the demonstration which derives from fewer postulates of hypotheses. Later, Robert Grosseteste (1168-1253), who greatly advanced the use of experimental methods in science, explained the principle of parsimony as “that is better and more valuable which requires fewer, other circumstances being equal.”

William of Ockham’s near contemporary Thomas Aquinas (1224-74) wrote that, “If a thing can be done adequately by means of one, it is superfluous to do it by means of several; for we observe that nature does not employ two instruments where one suffices.” Ockham advanced the discussion with, “Plurality should not be posited without necessity” and also, “What can be explained by the assumption of fewer things is vainly explained by the assumption of more things.” But the phrase ‘Ockham’s razor’ did not arise till 1852. Although the term is used to advance atheism, Ockham (being a Christian) believed in God as Creator, despite his criticisms of philosophical proofs for the existence of God.

However, when you say that Paul V. Spade claims that William "invented his dead-end nominalist philosophy," that is not the case as he elaborated a new form of existing Nominalist theory. He rejected the prevailing view the 'universals' really exist. He argued that they are simply artificial products of the human mind, necessary for communicating by means of language. Only individual or 'particular' things have real existence. Since knowledge was based on experience of individual things, natural science took on new significance.

Apparently denounced as a heretic to Pope John XXII, William was summoned to Avignon in 1324. There, a controversy arose about apostolic poverty, which made him more critical of the papacy. He called for a college of popes to rule the church, and claimed that Christ was the church's only head. He entirely rejected papal authority in secular matters.

Being "excommunicated for leaving Avignon without permission," as you pointed out, could be a vital clue as to the extent of bad feeling between him and papal authorities of his day. By then he had stressed that God was known by faith alone, not by reason or illumination, and that God's will was absolutely supreme. That would not have endeared him to the papacy, nor helped remove the charge of heresy. Perhaps, after such controlling tactics against him had failed and he was excommunicated, William of Ockham might have switched from being a sedevacantist to then being explicitly against the papacy per se. We do know that he fled, with the charge of heresy still threatening his very life, to the service of Emperor Louis of Bavaria, who supported him in his struggles with the papacy. William died in Munich.

As for him being "the first sedevacantist", the answer to that question would require deep knowledge of all Christians recorded in history from the time the papacy started until the time William of Ockham became a monk. My answer to that question is simply that he may well have been a sedevacantist (though eventually turned against the papacy per se before he died) but that many Catholics were sedevacantist either privately or simply unknown to Catholic authorities who never wrote anything about them. Given the immense hostility many individual popes aroused even in loyal Catholics, it is impossible for the long era of the papacy, until William's time, to have been devoid of sedevacantists. (Main source, article by Dr. H. D. McDonald, formerly Vice-Principal and Senior Lecturer, History of Doctrine and Philosophy of Religion, London Bible College.)