My personal theory as to why Opium devastated China more than other E. Asian nations is that in China, the opium sale was done on an absolutely massive scale, almost an industrialized level, that didn't occur on nearly the same scale in any of the nearby countries.

My focus was specifically on the economics industries of Hong Kong beginning in 1841 with Elliot's Proclamation where he declared HK would be the vested (economic) interest of the crown. It was a hugely important move for a number of reasons, but in my opinion it granted the British full control of a deep water port not far from the major transportation lanes on the mainland.

The British pretty quickly realized that they were bleeding silver due to the serious trade deficit with mainland China. They took a number of steps to remedy the situation (sale of monopoly rights, propping up smuggling industries, etc), with the largest being the sale of opium into China. These auctions of monopoly rights raised roughly ten to twenty-five percent of the colonial government’s annual revenue (Carrol).

At first, the British were directly selling the drug themselves, but overtime they realized they could make more by selling monopoly rights to different parts of the opium production process and the tax revenue on each of those steps.

Most of this all occurred during the first 10-15 years after the first Opium War, but by 1856, GB was again facing a trade deficit with China. Opium had become one of the most successful of the HK economies, so GB essentially doubled down on the sale of the drug to the mainlanders. This of course is where you can trace the roots of the boxer rebellion and the 2nd Opium War.

As I'm sure you know, the Qing were solidly thrashed during the 2nd Opium war, and at the Convention of Peking the Qing were forced to accept the full brunt of all war costs, guilt for starting the conflict, and fully legalize the sale of opium into China.

With full access to all of China's ports, the opium industry on HK absolutely exploded. Some sources say that by 1880 (roughly the height of Opium sales into China), Opium trade from HK was an estimated 7,000 tons/year.

I believe that answers the bulk of the period covered by your question.

Sources: I bolded ones that I think specifically answer your question.

• Naughton, Barry. The Chinese Economy: Transitions and Growth. Cambridge, MA: MIT, 2007. E-Book.

• Elliott, Mark C. "The Limits of Tartary: Manchuria in Imperial and National Geographies." The Journal of Asian Studies 59.3 (2000): 603. Web.

• United Kingdom & Qing China. Treaty of Nanjing. By Henry Pottinger, Qíyīng, and Yilibu. Nanjing: n.p., 1842. Print.

• United Kingdom. British Plenipotentiary in China. Proclamation of 2 February 1841. By Charles Elliot. N.p.: n.p., n.d. Print.

• Tsang, Steve, ed. A Documentary History of Hong Kong. Volume 1, Government and Politics. Hong Kong: Hong Kong UP, 1995. PDF.

• Carroll, John M. A Concise History of Hong Kong. Lanham: Rowman & Littlefield, 2007. Print.

• Ngo, Tak-Wing, ed. Hong Kong's History: State and Society Under Colonial Rule. London: Routledge, 1999. Print.

• United Kingdom & Qing Dynasty. China Foreign Relations. By Lord Eglin and Prince Gong. N.p., n.d. Web. 21 Apr. 2015. http://www.chinaforeignrelations.net/sites/default/files/1860%2C%20Beijing%20Conv.%20%28Br.%29.pdf.

• Carroll, John M. Edge of Empires: Chinese Elites and British Colonials in Hong Kong. Cambridge, MA: Harvard UP, 2005. Print.