No idea for koxinga's family and why it was chosen for dynasty.
About 1659 and not 1683, could you please explain and propose change for event?
About tech race, Manchus are really behind China in land. This is a fact. I have to check if Manchus are in same tech group as China (to be at least at parity) or suffer other penalties (number of known countries - isolation, for example). Btw, problem is the same for Japan and this penalty.
About spelling of chinese name, isn't Pinyin a romanization and not an anglicization?
I can't answer alone to the change you propose.
About relations, in order to avoid DA or at least delay them, maybe we could lower Chinese relations to the minimum "friendly" for Korea and Manchus, something like 50, maybe less (I can't check current values in 1419 right now).
EDIT: currently, we have in 1419:
China - Korea => +125
China - Manchus => +77
EDIT2: Manchus suffer isolation penalty, just like Japan (China techgroup in both cases, same techgroup as China in 1419).