Is there any external entity verifying the CO2 emissions quantities from China (and other countries) or are we expected to just take their word for it?

That would be landsat and its successors. Its pretty hard to hide climate at nation-scale. The leakage of Halons from Nth China was discovered by non-chinese.

I would be confident the actual CO2 emissions can be within some margin of error seen by anyone with a satellite with the right land sensing technology.

Where I diverge from you is the belief there is some systemic failure inherent in China to do what they say. They made the three gorges dam. They made space. They make Nuclear reactors When they want to achive the five year plan, they achieve the five year plan within limits: the backyard furnace thing back in the 1960s was a bit stupid.

China is complicated. it has provinces. Trump can rant about California but constitutionally cannot stop californians being californian. Because Federal-State is a thing. Well guess what: China is no different. China has provinces, and how well the provinces police this is about their federal-state structure, and is not evidence of systematic failure in the chinese model.