Ok so it's starting to come together:

1st to 6th centuries – the Barbarian-Roman dialectic

The Gallic Wars were a stalemate (rather than a comprehensive Roman win).

Therefore the barbarian mode of production Kropotkin described remained influential in France and Britain. A paper (DOI: 10.1007/s10814-015-9088-x) talks about how commonage is a pre-Roman influence on Roman and post-Roman Britain. The Visigothic Code (Spain, 642AD) combined Roman and Germanic law, showing that there was a dialectic between those two in that era. Germanic Law means popular assemblies and tribes (i.e. mutual aid groups) and compensatory justice (no cops, no jails). So that Roman-barbarian dialectic existed in Terra, and the Roman aspect led to feudalism. If we tip the Roman-barbarian dialectic to the barbarians, that has knock-on effects.

Because the Roman order doesn't dominate France and Britain, if Emperor Constantine converts to Christianity, it doesn't spread the religion. Gallic France, Anglo-Saxon Britain, etc remain pagan. Polytheistic paganism is a more compatible superstructure for a decentralised mode of production; monotheism is more compatible with worship-the-lord and tribute-the-clergy feudal production.

(I haven't talked about religion much, but the world is pagan rather than Abrahamic in case that wasn't obvious)

Class conflict in 'The Middle Ages'

This is when guilds start to emerge (in both Terran history and the alternate history). Let's look at some classes that exist and their class interests

Good guys for the purposes of our story here:

• Barbarians. People with a vested interest in the barbarian mode-of-production. Huns, "The Teutons, the Celts, the Scandinavians, the Slavonians, and others", the Russian mir. Their class-interest is to avoid enclosure, avoid anyone coming in imposing tax/tribute/surplus-value-extraction from them. The Chechens, the Turkic tribal confederacy, etc.
• Peasants. Their material interest is to keep production as independent as possible. An example is Terran history of where they succeeded is Friesland. Prevent enclosure.
• Craft guilds. Similar class interests to peasants really (the hammer to their sickle): keep production as independent as possible.

Bad guys –

• Aristocracy. They try to extract surplus value from the peasants and craftsmen. Where the post-Roman and feudal order exists, they exist. Where it does not they do not.
• Clergy. Pretty much the same as the aristocracy. They are much weaker in this world, not worth mentioning. (The Church is antagonistic to clans because they want people depending on the Church: "It has been argued that kinship ties were relatively weak in western Europe, perhaps as the result of deliberate policies by the medieval Catholic Church to weaken them")
• Merchant guilds. 'Bad guys' here because they are what became the bourgeoisie. Must smash. Their class-interest is doing things like enclosure

It makes perfect sense why the proto-bourgeois merchants, aristocracy, clergy have antagonistic interests to the barbarians and peasants.

Fitting the class guilds on to the side of the clans and against the merchant guilds is harder, but maybe I can make it work.

First problem with making the class alliance is that guilds don't historically side with clans: "guilds did not develop in the British Isles in the early medieval Celtic lands where kinship ties dominated.... Tine De Moor argues that weakened family ties were a vital precondition for the spectacular growth of guilds". Second reason is that craftsmen could be aligned with, not antagonistic with the proto-bourgeoisie, who could give them funding and markets. This is ameliorated if you remove the profit-motive: if the economy is moneyless, based on mutual aid obligations, the craftsman isn't interested in a bigger market. (The barbarian "blacksmith, who, like the blacksmith of the Indian communities, being a member of the community, is never paid for his work within the community" isn't interested in 'making more sales'.)

A series of wars between these two in the Middle Ages ends in the destruction of the merchant and feudal classes. One cool idea is it becomes a war-on-two-fronts for the emerging feudal and bourgeois system; they have Celts to the West (because Julius Cæsar failed to wipe them out), and to the East they have Turks, Huns, Chechens, the Nomadic Empire. A Celt-Khan vice-grip crushes kings.

The half-feudal-half-free people within what was the Holy Roman Empire – groups like the Old Swiss Confederacy, the Frisian Freedom, and peasant republics like the Republic of the Escartons – saw the writing on the wall and sided with the barbarian confederacy. That is in their interest. Basically all the mediæval people Kropotkin liked allied against all the mediæval people Kropotkin did not like.

Europe's contact with America and Africa in the Age of Sail

Contact is made between the confederated tribes of Europe and the confederated tribes of America. They both have a mode-of-production where they produce locally, rather than extract/exploit. They have no material reason to come into conflict. (In Terra, where they were all about extraction, they did.) You have democratic Europeans (democracies like the folkmoot and þing) meeting democracies like the Haudenosaunee. They start bartering and intermarrying a little. The dominant mode-of-production in Europe isn't exploitative, so instead of committing genocide Europeans start wearing moccasins because moccasins are comfy as fuck let's face it.

Similarly, plenty of cool, chill societies in Africa like the Igbo that are based on mutual aid. "It is therefore obvious from the way societies like the Tiv, the central Igbo, and the Dagaaba were organized that they were well aware of the political structure of the centralized systems, but tried to eliminate them as much as possible.... such ethnic societies as the Tiv and Igbo of Nigeria, the Nuer of Sudan, the Somali, and the Bedouin Arabs throughout North Africa..... In general there were no officeholders; only representatives of groups.". So when Europe and Africa start making more links (in the 1400s), it is European tribal confederacies without an extractive economy, and without a religious imperative to convert/subjugate heathens.

Europe's contact with India and China

Now the above doesn't explain why East Asia would follow the same pattern, but a similar thing happened in Terran history: the treaty of Westphalia established nation-states and later the entire globe was nation-states. Similar here but with tribal confederacies.

The Great Divergence

The guilds want to train apprentices in every newly-contacted country to spread their influence. This serves as a technology-transfer mechanism. Industry is not nationalistic: there is an inter-national transatlantic class of engineers: the guild. It is into that world that the steam engine comes. Technology doesn't give Europe a competitive advantage, because technology gets spread.