Thing is the Celestial Dragons aren't just people, they are overall portrayed as villanous, and Mary Geoise is a place built ex profeso for them, not occupied by an outsider; if anything, the role you're giving to the world royals would belong to lunarians.
Mary Geoise wasn't built for CDs, it already existed before They lived there
It's 20 OG Kings who went to Live there & no one said that They invaded it
Actually if They took it by force to become WG, then Nefertari would have been included in that Invasion
But it's clearly stated that Nefertari did found WG, but didn't go to live in Mary Geoise
Meaning WG was Established & People accepted it before those Kings even set foot in Mary Geoise
Why would They need to Invade it if They are already appointed Rulers of The World?
Seeing it's Design & Castles, it's clearly that it has always been Home of Knights & those Holy Knights are probably Descendants of OG Inhabitants who vow to Protect Order
There is nothing that says Lunarians lived in Mary Geoise, actually WB hinted that it didn't even exist when They had their own Kingdom
And Red Line is Huge, it's literally as Big as Grand Line, nothing says that They were living in same spot as Mary Geoise
Also if you look at all Locations that Luffy Liberated, Oppressed People were Living in said Location
So Oppressed People of Mary Geoise War must be already Living there, even if in some Corner
Not completely far from it
Also Oda created a Land for Samurai & Land for Vikings, it's obvious that Knights also have their Own Land
Not to mention that Main Villain of such Arcs is Known, not Hidden like Imu
Morgan
Buggy
Kuro
Krieg
Arlong
Wapol
Crocodile
Enel
Moria
Hody
Caesar
Doffy
Kaidou
These Villains are known to anyone who lives on their Island or Visits it
Regardless if seen as Heroes or Villains, but Imu is Hidden, even those who work in Mary Geoise don't know about him
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I mean Mariejois is going to be destroyed when the red line is destroyed so… your reasoning here doesn’t work
Red Line is many Islands Connected together
It's not like it will Disappear from Existence, it simply will be Split into Pieces & turn into countless Islands scattered around the World
So Mary Geoise doesn't necessarily need to be Destroyed when Red Line is shattered, it will simply turn into an Island