Since nobody gave any answers, I’ll try to offer something helpful. I’m far from an expert and will be using general sources, Ockham’s detailed entry in the Stanford Encyclopedia of Philosophy and the not quite as detailed but more accessible entry in the Internet Encyclopedia of Philosophy. There’s one really good thread about him from this sub, an AMA with Ockham expert Rondo Keele, and it’s helpful for setting the context for his thought in that conversational kind of way. I hope that this will be good enough for the mods to keep around.

So anyway, I’ll answer your third question first because it’s the most straightforward. It’s fairly simple to show that Desrosières is mistaken about the relationship between Ockham’s “razor” principle and his political arguments, due to the basic facts of his biography.

Ockham’s career can be divided into two very different periods. From 1317 to 1324, he was writing and teaching in England, first in Oxford, then in the Franciscan convent (Greyfriars) in London. In this relatively brief period, he wrote on a wide range of the philosophical topics of the era: logic, metaphysics, theology, psychology, ethics, natural philosophy, philosophy of language. All of these writings exhibit the razor principle in different ways. Ockham sought to show that much of Scholastic philosophy had been needlessly complicated, invoking metaphysical entities that were not necessary.

Then he was called to the papal court at Avignon so that his teachings could be investigated for heresy. He continued to write while there, finishing works that he had begun in England. Ultimately he was not found to be a heretic. But in 1328 he got caught up in another controversy, the conflict between Pope John XXII and the Franciscans that you alluded to. Ockham wrote to defend the Franciscan position, in the process arguing that John was not only wrong, but actually heretical - and therefore not a legitimate Pope. Needless to say, this got him in real trouble, and he fled, with the head of the order and a few others, to the court of the Holy Roman Emperor Louis IV, who was involved in his own struggle with the Pope at the time.

From then on, Ockham wrote exclusively on political topics, developing a theory not terribly unlike what we might call the separation of church and state, though it was framed in terms of Pope and Emperor and did not question basic medieval assumptions about hierarchies. He stayed in Louis’s court for almost twenty more years, dying in Munich in 1347.

So you can see that just about everything Ockham wrote that really exemplified his razor happened before he got involved in that dispute between the Franciscans and the Pope. By the time that issue took up his attention, his razor was well established, and indeed it did not play a major role in his political arguments.

That hopefully answers your third question. It’s maybe not surprising that a historian would seek a political motivation for the famous Razor; but it’s not supported by the chronology of Ockham’s writings. (more in next comment)