How would China's IQ scores compare if the effect of widespread endemic cheating was considered?

*Correction America did make the top 15 once in Science in the 2000 PISA.

My apologies

Multiculturalism. More specifically immigrants and their children who are loud, violent and disruptive.

Also the lack of meaningful punishments plays a role in here.

Yeah, no. There are not enough immigrants that would account for a drop as high as you claim. Do the maths. Post your sources.