I forgot to add another retcon explanation/flesh-out, kind of a big one:
The Sack of Rome - Rome is indeed sacked in a manner similar to OTL, but in 1512 rather than 1527. This sack is carried out by unrestrained French gendarmes and Swiss mercenaries instead of Charles V's underpaid troops. During the Second Italian War, a French army under Jacques de La Palice pushes deep into Lazio after routing the Papal army under Prospero Colonna at Viterbo and soon finds itself within range of the Roman suburbs. With no force ready to defend it, Rome is a very tempting target for the French and Swiss, who are (of course) underpaid and fairly irked that over the length of their campaign. Similar to OTL, there's also a widespread, simmering distaste for the corruption of the Papacy and a bit of patriotic hatred for Pope Julius II's "Fuori i Barbari!" anti-French + anti-Swiss + anti-foreigner foreign policies, so the thought of giving Rome a whipping is appealing to even the more devout members of the Franco-Swiss army. When Jacques de la Palice unexpectedly dies from a camp illness, his lieutenant, the Scotsman Robert Stewart, is unable to maintain control. However, since this is before the Reformation, the sack is still less religiously charged than IOTL and while atrocities are committed, they are not quite as numerous and appalling either. This damages the reputation of the French monarchy and is one of the reasons why the French are so isolated in the later 20 Years War (having to ally themselves with the Protestants), and when the French eventually turn around to free Florence from the pro-Papal Cesare Borgia, their defeat there is seen as the anticipated punishment of God. This has a serious effect on TTL's Protestant Reformation. As some of you may have noticed, TTL's Martin Luther is a little bit more subdued in his anti-Catholicism than IOTL, and this is rooted in the 1512 Sack of Rome: having done his pilgrimage to Rome in 1515, Luther sees the long-term effects of the depredations done there three years earlier and is therefore not exposed to an overly venal, debauched Church as he was IOTL, and his primary issues are limited to paid indulgences, simony, nepotism, illiteracy and an overall lack of quality control in the priesthood, and the need for a greater focus on faith rather than works in relation to personal justification. Even Luther's criticism of indulgences is a little less excoriating ITTL, as most of the money accumulated goes to repairing Rome's sack-related damages. Since the Habsburgs ITTL are limited to Central Europe, they have a better finger on the pulse as to that's going on in Germany and therefore choose a much more sensitive approach to Luther and his associates - going so far as to block the entrance and dissemination of papal bulls that announce Luther's excommunication or emphasize his heterodoxy. Luther never fully re-enters the Roman fold, but his followers are much less separated from the Catholic Church and this "Saxon Rite" Catholic congregation more or less remains or re-enters into communion with Rome. Due to the cooperation of the Habsburgs with Luther, you could even say that Charles V was technically a Lutheran ITTL (albeit following our different definition of Lutheranism). Johann Meyer later springs up to fill in the gap left by Luther's closeness to Rome, as Luther's arm is held very tightly by the Habsburgs and there are many people in Germany that are too agitated to stay in the moderate camp. This earlier sack is also one of the reasons the Papacy ITTL is so willing to address the need for major reform and call an ecumenical council. The OTL 1527 Sack of Rome was a major wake-up call for a Papacy that refused to directly devote effort to reform and answer the complaints of its detractors ( Pope Clement VII stated that an ecumenical council should be avoided as long as it could, for instance), meaning that ITTL the Papacy has received this wake-up call 15 years earlier and before the Reformation instead of 10 years after it was already underway.
Again, let me know what you think, and if you have any questions.