I don't think there's any evidence of a global compromise but I think you're right that China wouldn't allow access if it didn't ultimately control it.

I couldn't find anything specific about iMessage but the keys are backed up to iCloud -- and we know that's compromised. I can't imagine them leaving users the option to just not back up to iCloud to avoid surveillance, but I haven't seen any specifics. Best to assume that under no circumstances do you ever have privacy from the gov't in China or even when messaging someone in China.