It never occurred to me that Nii might NOT be the major villain in the series, but that would certainly be a very interesting twist.

I've been wondering that too-- if Nii's just a midboss after all.

Now the question is, did Nii/Ukoku plan Koumyou's death, or was it outside of his calculations?

I've always thought of Koumyou's death as a Queen's sacrifice to protect the King. Which with Nii playing chess all the time works pretty well for me.