I don't know about the Panama Canal, but the rest is true (mainly UK).

" ... transporting porcelain and whatever other goods China exported at the time."

China exported Silk, porcelain and tea like the is no tomorrow but did not buy anything. At one point China accumulated a huge part of the world silver reserves, sucking liquidity out of the western economy. Opium "solved" this problem.

It was my understanding that the Opium trade was mainly a GB thing. I was not aware that the US was involved.