The heyday of common law and privatized law which Nick spends so much time on was well before the 1500s and 1600s

By "privatized law", you probably mean jurisdiction as property, which of course is no more in England or in any other country. But on his blog, he spends quite a bit of time explaining other aspects of common law which continue to apply to this day.

property rights & institutions were often as good or poor in various eras of China. This would suggest that they have small effects, but aren't all that important in the long run.

This does not follow. Even if (as you suggest) property rights were "as good" in China as in England, there are many other features of a judicial system not captured by the phrase "country X had better property rights than country Y" that might have made a big difference. For example, starting around the time of the Glorious Revolution IIRC the English courts insisted that even the King of England had to pay his debts, as described in one of Nick's blog posts, and I severely doubt that any court in China ever dared to rule against the Emperor of China in that way.

I have not read Farewell to Alms. Is not his thesis that the unparalleled economic success of England (and certain spinoffs from England, like the American colonies) in the 18th and 19th Centuries came from the fact that the English people had been subjected for a longer time than any other nation's people to selection pressures for capital accumulation and success in capitalists enterprises in general. If not institutional differences, what if anything does Clark say created the earlier and longer selection pressures? Is it just that England, being an island, was invaded less often than territories on the Continent?

Anyway, to this day, the English-speaking countries, which, with the exception of India (which you can almost regard as a medium-sized English-speaking country inside a much larger country) are the only countries using common-law judicial systems, are better at creating wealth than any other countries, even thoroughly Westernized ones like France. I am of course not claiming that India is better at creating wealth than non-English-speaking Western countries. Average IQ is drastically lower in India than in any European country, and no choice of institutions could have conceivably overcome that handicap to economic performance. (Moreover, India's judicial system departs from the English model in some ways, e.g., they have done away with juries.)

Correlation does not alway entail causation, but when a correlation between economic performance and whether the country used the English model for its judicial system persists for centuries, well, that's evidence for causation in my book. Note that the scientific knowledge created by Newton, Hooke, Darwin, etc, were readily adopted by other nations (as was of course scientific knowledge created by other nations readily adopted by England) and the rest of the world was eager to adopt the methods of the industrial revolution (Germany being only 30 years behind England in industrialization, IIRC) but to this day, in France for example, what the French call "the Anglo-Saxon model" for governance and the economy is considered too harsh and inhumane for adoption in France. The only other feature of English society whose adoption is opposed strenuously in France is the English language, and I severely doubt that the English language is the cause of the superior economic performance of the English-speaking countries relative to France.

I readily concede that everything I wrote above is guesswork and could very easily be wrong. But given how important economic performance is (and given the potential for insights into the causes of superior economic performance to transfer to other domains of human effort, like creating better online conversations) guesswork is probably worth communicating, if the communication can be done in an honest, equivocal manner like I am trying to do here. It does not take much reading about the differences among judicial systems, and it does not take much experience with the operation of an actual judicial system (particularly if one manages or operates a business) to come to believe that the characteristics of the judical system tends to have a large effect on the economic performance of the community served by the judicial system.