Yeah, I believe some of the ancient China kingdom are multicultural state. Chu state for example I believe is initially homogeneous state, later on they absorb other culture and become multicultural state as they expanded south and east.
It wasn't just Chu and the concept of diversity wasn't fringe. Other states were no more initially homogeneous and then progressively multicultural as they expanded in their various directions to include their own indigenous elements of culture. The argument could be made to say that it was Qin that was most multicultural since it imported elements of culture in every way from all around ranging from architectural to burial to philosophical. Qin thereby made the core of many Chinese systems available to themselves and co opted them in ways that would have only occurred in their own domain since they also had a militaristic culture central to their society.
Indeed the same can be said for the Yue state because it conquered Wu and parts of Qi I believe.
I wonder if the same can be said about Yue state, which explain the influence of both Miao/Hmong languages on toponyms and the Chinese writings found on the sword of Goujian.
Regarding the writing system, I believe it is safe to say the ancestors of modern Han Chinese are the one discover since they are the one who inherits it and still using it in modern day.
While it is safe to say the ancestors of modern Han Chinese are discoverers of Chinese script this just isn't a really meaningful way of looking at it. Which is why to say Han Chinese are called as such, by the metric of Qin clerical Chinese script, only conceives of a splintery fragment of all the things available in China to which can be issued into correlation with the Han. For example we could also name the Han Chinese for their love of silk, painting, and Hanfu which has nothing to do with the script which originated in areas that could produce no silk. Or we can name the Han Chinese for their 4 great inventions of paper printing compass and gunpowder which saw very high use during the time of Tang and Song dynasty but by these times a great majority of everyone in China had already become a member of Han Chinese. It may be more or less meaningful depending on how you look at it. The point is many of these splintery fragments which end up constituting the Han whole are derived from many diverse regions containing ethnic groups each once having had an identity to call their own.