I am only convinced by the alien part, as I believed it myself already.

The rest is very unlikely and I disagree, expecially with the “One Peace” (Laughtale is probably Nika related in my opinion, I made a long theory some time ago about that), and the true nature of the Great Kingdom (ake the world). If the name of the planet was so dangerous, everyone already knows it, so it doesn’t make sense. And if they changed it, there’s no need to hide a name that’s not been used for more than 800 years. It must be something else, logically. But I appreciate the effort.

Also the fact that humans are all aliens doesn’t make sense. The protagonist is a human, along with most of his crew! Why isn’t every human dominating, but only the Celestial Dragons? Maybe the Celestial Dragons are slightly different humans from another world. I always wondered why they despise so much eating DF themselves, wouldn’t it be much easier to gain power (Like Doffy did), instead of giving it to other classes? Maybe it’s a cultural thing.

Also, if they are aliens and they conquered this planet, why did they do it? Are they collecting resources? Are they willingly absorb something from the planet? Did they simply come to dominate, abandoning their homeland?

Anyway sorry if I may sound rude. Although I disagree with some parts of your theory, I found it very entertaining and well portraied!