"So the only measure left is to get Chinese police to put a stop to the killer chemical from leaving the factories where it is manufactured."

Or... legalize and regulate heroin, making it available in a safer form. The reason for the surge in fentanyl and related deaths is that drug cartels profits collapsed as US states keep legalizing cannabis, so they resorted to pushing this stuff into their channels. That is instructional, in that the vast financial and social costs associated with the war on drugs can be far better put to use toward regulation and harm reduction strategies.