Historically, how did Protestantism first arrive at a consensus?

You could say that at any one moment in time the Roman Catholic Church (usually) has just one Pope, and Protestantism has none. But it may be better to say that Catholics only have one Pope between them, but Protestants have one Pope each.

For some "Protestant" simply means you are not Roman Catholic. Other Protestants value the freedom to follow their own path, others value the freedom to interpret Scripture without fear of rejection from some religious authority. The truth is the term is so woolly as to be almost meaningless, unless you take it in its traditional sense - the name for those who are protesting against the errors they perceive exist in Roman Catholicism.

It seems to me you have much too great a concern for "consensus". During the (largely Methodist) Evangelical Awakening of George Whitfield, John Wesley, Jonathan Edwards, etc, there wasn't doctrinal consensus between the Whitfield and Wesley - one was Calvinist the other Arminian - but the Lord blessed both and there was a great turning to God nevertheless.

It isn't agreement amongst ourselves that is needed - it is the godly living of believing prayer, lots of faith in our God and Saviour Jesus Christ and love for him, reading and submitting to Holy Scripture, zeal, and lots of love for the lost. It is personal agreement with God that is needed.

The scripture you quote shows that there were divisions amongst Christians circa A.D. 55, as existing divisions prompted Paul to write that admonition. He goes on to mention the risk of sectarian splits with some following Paul, Apollos or Cephas. Others wisely said they followed Christ. In chapter 3 Paul speaks again of jealousy and quarrelling and factions, then in chapter 5 he exposes gross immorality in their midst. Yet, despites those disagreements, the first century Christian Church existed. The same could be said of the newly Reformed Church in the 16th century. It existed despite disagreements and sectarian splits and immorality. But your question is about ‘consensus’ regarding Reformed doctrine; you want to know how it was arrived at. Let me quote the start of the book ‘The Reformation’ by Owen Chadwick (Vol. 3 of The Pelican History of the Church, 1979 edition)

“At the beginning of the sixteenth century everyone that mattered in the Western Church was crying out for reformation. For a century and more Western Europe had sought for reform of the Church ‘in head and members’ and had failed to find it. If you asked people what they meant when they said that the Church was in need of reform, they would not have found it easy to agree.” Page 11

It can safely be said that the Reformation began despite disagreements as to what, exactly, needed to be reformed, and how. We know that Martin Luther’s 95 Theses lit the touch-paper that caused an explosive result, with reverberations down to this day. Yet, bear in mind the extent of doctrinal issues therein, compared with other matters. Quoting again from Chadwick:

“When churchmen spoke of reformation, they were almost always thinking of administrative, legal, or moral reformation ; hardly ever of doctrinal reformation. They did not suppose the Pope’s doctrine to be erroneous.” Page 13 “The Ninety-Five Theses contained no mention of the doctrine of justification by faith. But in spite of the silence of the theses, [Luther’s] attack upon indulgences sprang out of ‘my theology’, out of a Pauline conviction of God’s grace. The Indulgence he believed to be pernicious because it was misleading simple souls. He saw it as an external and damnable symptom of so much that was inwardly wrong with the Christian teaching of his generation, a teaching which asserted or suggested that God could be placated by external acts, by forms, by payments, by ‘good works’. Luther did not attack indulgences and thereby reach a doctrine of justification by faith alone. He applied an already appropriated doctrine of justification to judge a particular indulgence.” Pages 46-47

I mention all this because herein lies the understanding of what role ‘consensus’ played both in the Church in Corinth, and in the Reformation. When it is clearly seen that something is very wrong in the Church, the need is not to get everyone round a table to agree to changes in administration, or to vote different people in to office, or to produce a list of doctrines from which to form some Creed or other. The need is to identify from Scripture alone the principles that are being violated by the Church, then to lay the axe to the root of that problem. It’s no use trimming the shrubs if they are still hiding the root of the problem. When Luther nailed that paper to the door, it was the first nail in the coffin lid of a corrupt church system. It wasn’t a theological treatise. Theology got thrashed out later.

Huldreich Zwingli, the reformer of Zurich wrote “Commentary on True and False Religion” (1528), a systematic theology with impact. He rejected much of Catholicism, Lutheranism and Anabaptism. (Luther also turned against Anabaptism.) But Calvin’s “Institutes” formed a general consensus. He died in 1564.

By the time the Reformation really got going, the consensus was not about church systems or secondary matters that Paul identified as “disputable” (Romans 14:1) – it was based on a rediscovery of the power of the unadulterated word of God, that was now being translated into the vernacular and put in the hands of ordinary folk in printed form. And that is what needs to be rediscovered today in Protestantism.

“Always Reforming” is the motto many Protestants are supposed to go by. Yet Reformed Christians keep hiding in their comfort zones, not even looking at the modern issues that are corrupting the Church today. How many see the “external and damnable symptoms of so much that [is] inwardly wrong with the Christian teaching of [our] generation”?

The basics of essential Christian doctrine have a large consensus amongst Protestants. But the need is to live them out in everyday life and in society, showing to a godless world the transforming power of the word of God in believers’ lives. It’s not “contradictory denominations” that is the problem today, but “contradictory living” by those who claim to be “Reformed” but whose profession could be doubted on the basis of their worldliness and their love of church systems. The need is to promote the biblical gospel, and to live it. “Come back, Luther! All is forgiven!”

The Reformation beginning in the 16th century was actually three different movements consisting of: (a) Calvinism (b) Lutheranism (c) Anabaptists. There were conferences between these groups but the only things they were united upon were:

• Opposition to the authority of the Catholic church largely due to its excesses and abuses
• Salvation by faith alone (the five "solas" actually came later)
• Doctrine based on Scripture

However, there were numerous fights between these groups. For example, Calvin persecuted Lutherans and Anabaptists by having some executed, etc. Conferences between Luther and Zwingli ended in stalemates. There were on-going debates particularly about how communion should be practiced and what it meant, which persist today.

As time progressed, further subdivisions emerged such as "enthusiasm" (the basis of the charismatic movement) and many more. Thus, the reformation was never truly united.

Good overall summary: https://en.wikipedia.org/wiki/Reformation as well as https://en.wikipedia.org/wiki/Anabaptism