You might want to reread Koala's/Fisher Tiger's backstory. Obedient, fearful citizens shut their mouths when they feel it's the safer choice for them. Most characters are not as courageous as the ones Oda's story focuses on.
Stories in one piece spread by word of mouth and often get distorted. And the majority of the One Piece citizens are not pirates. Most people have no idea about what's even going on in the world of piracy and what the world government really is. If you were an average one piece citizen that fears the world government, would you believe the testimony of criminals,pirates or revolutionaries ? Would you interact with them in the first place? Do real world citizens believe every conspiracy theory spread by unsavory in individuals or those considered political extremists?
Hey, so while I respect your opinion, I feel like you're trying to excuse writing that simply doesn't make sense. For example, while many of the people who escaped God Valley were criminals, it was also shown that many of the characters who escaped were slaves. We have characters like Kuma who went on to become well respected members of their communities and was shown to be a beloved member of his new community. I find it hard to imagine he was the only one. We can also look at other characters who used to be slaves of the CD's like Fisher Tiger and Boa Hancock. We could say these characters are pirates, and ofc they are, but they are also well respected individuals by common citizens of their islands. These citizens at some point likely talk to people from other islands... Again this is how stories spread in OP, even things like Nika/Joyboy etc did and have spread no matter what the WG tries.
Also, for example someone like Whitebeard who was at God valley despite being a pirate had enormous political influence. Many people in the world looked up to him and appreciated him for protecting islands like Fmi. Again, are we really supposed to believe WB had some reason to cover this up? And then we have to factor in the Marines/Cipher pol. We have many characters from these organizations who eventually left and did other things. Most famously, we could say Dragon. But there is obviously other Marines who have families they would tell, or might simply just switch jobs(I don't think its ever been implied Marines have some lifetime contract). We can even look at characters like Garp who has canonically been shown to constantly slip things through and by mistake revealed that Dragon is Luffys dad.
Then we have the Roger pirates. Remember, when Roger found laughtale, he spread the word, and Big news Morgan's himself reported on him finding the last island. Why couldn't Roger do something similar about God valley? People may believe it or not, but at the least, everyone should know about the celestial dragons existing, and from my understanding Luffy and Zoro didn't. Then we would just have to point to these countries suddenly becoming inhabited every 3 years.... God valley was in the west blue, which are generally considered more peaceful compared to the grand line... This should be a very shocking event, and we have to believe that no one put the dots together that the Marines were traveling to the west blue the exact time all these people died? No countries traded with God valley or any of the other countries that lose their entire populations every 3 years? As far as we know, there's not even an official explanation for any of this.