You said ....it turned out these Mexican coins were circulated in China as currency. that maybe also explained why "Why did they choose to counterfeit a common Mexican silver coin".
The Mexican 8R was a true WORLD
trade dollar during and AFTER it ceased to be used for money at home.
The 8R is the standard for the Silver Dollar adopted by the US constitution and it was the economic basis of world stock markets. I doubt that when considered over all that any other coin has ever surpassed the 8R as a standard. As such it was also one of the most copied coins in the world.
I know of over 600 different counterfeit die varieties of the Portrait 8R alone. The number of forgeries of the Cap and Ray 8R is far higher - nearer 3,000 varieties now known.
It was also the dominant dollar coin used in the US until 1857 as attested by JL Riddell's 1845 estimate that 90% of all circulating coins were Spanish American or Mexican. It was current in Japan until 1893, the East Indies until 1895 and China until 1933. So it was counterfeited in many different countries at many different times - for DIFFERENT reasons.
It is one of the few coins that has been forged during the original period of circulation both as a "Debased" and as a "Full weight silver" forgery.
The Pillar and later the Portrait 8R coins were Colonial Era coins forged from the 1730s until the 1820s. It was as part of the War effort against Spain (1796) and later was forged as a "token" that saw use in many British colonies (ca 1795-1820). In the US forgers made 8Rs for about 3 centuries and only stopped in 1933 when they were demonetized in China.
So, yes, Common 8R coins were forged for every day use in commerce, as Tokens, as bullion forgeries to dispose of Comstock silver when the US stopped making dollars domestically and most recently as Numismatic forgeries to take advantage of Novice collectors opf coins and bullion.
I hope you can see why I love the series.