I guess Boeing doesn’t have leverage over China to force them to cooperate with the investigation? They need the China business as much as China needs their planes?

I don't think it is because of the business aspect; Boeing in any investigation won't have leverage as the investigations are run by the local equivalents on the NTSB. To the extent there is any leverage, it's the NTSB that might have it, but it would be informal. The Chinese government is stonewalling this because they deem it politically inconvenient/embarrassing that they had a pilot suicide/murder take place. For instance see also https://admiralcloudberg.medium.com/the-crash-of-egyptair-fl... for the story of EgyptAir 990.