Questions about example sentences with, and the definition and usage of "Zheng"

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A: × In 1405, the leaders in the Ming dynasty set Zheng to the Western Seas in order to border defense and start the business over sea.
✓ In 1405, the leaders in the Ming dynasty sent Zheng to the Western Seas in order to defend the border and start the business overseas.

× During the later 28 years, Zheng led the ship teams which the number of members reached to roughly 100,000 people to the Western Seas seven times and visited 30 countries and regions or more.
✓ Over the next 28 years, Zheng led the ship’s team (with the number of members reaching roughly to 100,000 people at its peak) to the Western Seas seven times and visited 30 countries and regions, possibly more.

× The team reached to Africa via Southern Asia and Western Asia.
✓ The team reached Africa via Southern Asia and Western Asia.

× Zheng visited the Western Sea is an achievement in history of the world navigation, it displayed not only Zheng’s remarkable ability on navigation and leading, but also the Ming dynasty’s national strength and prestige, and strengthened the relationship between Ming dynasty and countries over sea.
✓ Zheng visiting the Western Sea is a major achievement in the history of world navigation, it displayed not only Zheng’s remarkable abilities in navigation and leadership, but also the Ming dynasty’s national strength and prestige. This helped strengthen the relationship between Ming dynasty and many countries overseas.

This overall is very well translated in my opinion, just a few issues that hinder the fluency to the reader, apart from that well done!