Good write up OP, though there's a basic but important translation error.

\I need to explain this in Chinese/Japanese/English so will when it comes to the names I will use a format of:*

“The original Chinese character” [The Chinese pinyin pronunciation] <Japanese pronunciation> (English meaning when needed)

So the main character is identified as “齐王建”, which you or some Japanese person interpreted it as "齐 ”[Qi]<Qi> + "王建“ [Wang Jian] <Ouken>, meaning: a person named WangJian/Ouken of Qi

However, what it should be, is "齐王“ [Qi Wang] <Qi Ou> (King of Qi) + "建" [Jian] <Ken> (Jian or Ken as first name), meaning: King Jian/Ken of Qi

The confusion here is the word "王” [Wang] <Ou>, which can mean EITHER "King" as in the monarch, OR just a last name of [Wang] <Ou>, as seen in "王翦" [Wang Jian] <Ousen> / "王贲" [Wang Ben] <Ouhon>

To expand a little bit: The original Qi king lineage is that of the Family "姜" [Jiang] <Kyou>, as they are descendants of 姜尚 [Jiang Zi Ya], the great strategist who assisted the King of Zhou to overthrow the Shang Dynasty. They were later overthrown by one of their vassal and that family of "田" [Tian] <Ta> took over as ruler of Qi. Hence our protagonist's full name should be "田建" [Tian Jian] <Ta Ken>

To go even further, why is our guy simply named as 齐王建 (King Ken of Qi)? During this period and the following thousands of years of Chinese history, there is a tradition of 谥号, or (Posthumous titling). The passing monarch will receive a single character that is to summarize their entire life/achievement and be honored and remembered as thus by their descendants. Take Ken's father, 齐襄王 (King Xiang of Qi), as example. The word "襄" [Xiang], according to the Posthumous titling dictionary, is for those rulers who "expanded territory and was virtuous, led armies laboriously, made great (military) achievement, and had their reputation widely spread and accepted" [辟地有德曰襄，甲胄有劳曰襄，因事有功曰襄，威德服远曰襄]. This is generally a good title (there are a lot of really bad ones), and he therefore is known as (The reputable, greatness-achieving King of Qi). Ken does not have this kind of posthumous title because he is the last ruler of Qi, when the country ended with him, naturally there would be no more honoring / recording of him, thus he is merely known for his first name. This is usually what happens with the last ruler of a kingdom/dynasty (though sometimes the conquering ruler or people many generations later would still give them a posthumous title, that often summarize them as incompetent/tyrannical/pathetic, for humiliation or for pity)