If China wants a base on the Sea of Japan, I'm sure Kim Jong-un would let them built one on the coast of NK.Another No.
I think Pecksmith you need to re-focus. Look at Sino Russian history and Geo politics but think much longer term. Think like the Chinese. If and as Russia weakens militarily and economically, the Chinese will seek to take advantage of opportunities as they occur. It might take 20-50, maybe 100 years, but they will be ready. Siberia is no big deal but China with a base in the Sea of Japan would be massive for China, Russia, Japan, Korea and the USA - but not gunna happen any time soon.