@VitaVitaARThe council of the united nations did not reveal that information to the rest of the world, meaning that revealing it to the united nations at most is revealing it to just the upper council. It would be top level government secrets at that point meaning that mystery is still maintained. There is a reason why there was such a huge fallout and coverup after the events of the part 1 of the singularity.
Reminder that Chaldea is for all intensive purposes STILL a secret organisation, especially due to the fact of what the CHALDEAS system is, any companies it has relation to are either under false pretense. E.g pretending to be something else. Or only the upper levels of those company know. This is still billions of less people then the entire world. So mystery is unaffected
Mystery is not negated in FGO, in fact many things still use mystery as an explanation for their potency.
2. Riots would likely never end considering these are servants that could take out entire armies by themselves as all of them are superhuman. Even the weakest one can kill several men without a scratch. Knowing that the magus population is less then 5% of the entire world. This would cause a inhuman amount of civil unrest for generations.
This is only worsened by the fact that world-buster servants exist like Gilgamesh which if public knowledge of that exists would likely cause many nations to live in fear of the man that can end the world in a single instant if he so tried. This creates a cold-war scenario where likely the nations would want to eliminate such powerful servants. Because nobody is going to get a good nights sleep if they knew a kid with three command seals could wake up tomorrow with a world cracker and have a bad day that results in the entire world going boom
3.
Besides the logistical problem of the mystery being completely ruined by this island bringing attention to it. Setting up an island state as an independent nation of servants, while also letting servants exist in such close proximity to each other, when most of these servants (logically) would want to return to their homelands. Would cause such a massive war of culture. But that isn't the main issue.
The main issue is painting a huge target. The nations of the world don't have servants which are stronger then a nuke. but this one place has hundreds of them. How can you as the next leader of your country assure national security when say... Arash could walk into your city the next day and stella the entire thing to the ground?
Tidbit: Elementary my dear (sherlock holmes NP) that solves all mysterys is anti-world because of what it does to mystery (in a magic sense), which means Mystery is very much a key factor still.
