It seems weird indeed, but it doesn't seem that this knowledge is really that accessible. Yes, everyone knows the history of WG's foundation, everyone knows that there were 20 kings, but their surnames actually seem to be in a fog. Out of the plebs, nobody besides Ivankov, who probably has the best intel in the world, ever commented about a CD's surname. The CD families don't have that much exposition, maybe just the royalty and the nobles of the WG nations know about it (and there's ignorance even among them, as seen with Sterry). Up to this day, we still don't know Rosward's, Charlos' and Shalria's surname, and they seem to be one of the most public CDs out there.
About Luffy and the "Gomu Gomu no Mi": the information in One Piece also takes some time to arrive in Marineford/Mary Geoise. I don't think that they could've captured Luffy before Water Seven (W7 was really the first mistake IMO, as the strawhats stayed there for 3 days after Enies Lobby). They stayed for 3 days in Alabasta too, but they were hidden in the most unlikely place: Alubarna's royal palace. After W7, the WG did mistake after mistake, but that's due to the Nika retcon (it make it seem that they didn't really care that much about Luffy or the Gomu Gomu no Mi in the past). Yes, the Sun God Nika was one of Oda's biggest blunders IMO.
