Retaining Hong Kong would not reduce China to North Korean stagnation

Would China stagnate like North Korea if the United States stayed out of Suez and the Commonwealth retained Hong Kong?

No. Hong Kong was important, but China's wider economy and industrial base were too large for those changes to produce that result.

The counterfactual gives too much causal weight to Hong Kong and the United States' Suez policy. Hong Kong mattered to China's economy, but the country's economy and industrial base extended far beyond the territory. Continued Commonwealth control would therefore impose a political and strategic problem without erasing China's wider capacity. American abstention at Suez is judged largely irrelevant to the proposed economic outcome.