1) I think the motivation of the people running the race is going to be part of the long term subplot. To get away with it is has to be a really powerful conspiracy. Bored elites like the original Hellfire club? A wilder speculation in #3
2)All the "random" people showing up are not random. Either they work for the people running the race, or they work for the "players". (It is pretty obvious that the racers are game pieces. Each team has a "sponsor" who lures, coerces or manipulates them into taking part; There is some heavy betting going on. )
3)Maybe the great race started out along the lines of a story I read once Brek-brek-brek-Koax-Koax where world history was being decide by lunatics in an asylum racing frogs. The race is surrogate warfare. The world elites, large governments and militaries get together and set up the race as competition to avoid actual shooting wars. To avoid the whole problem that you had with the Trojan war, where the loser does not accept the results, the racers and their circumstances are carefully setup to be analogous to the actual strengths and weakness of the groups they represent. It is both the ultimate conspiracy story and also the ultimate critique of conspiracy stories in the sense that any conspiracy that produced the actual world we see would have to be a conspiracy that made no sense. Wandering off track I know a cross between fanfic and speculation. It is only to avoid really big wars WWII level or nuclear armegaddan - that is wars where the big boys take part directly on both side. Small wars, and even big wars where you don't have troops from the big powers fighting each other are still allowed. But other sponsors have muscled their way in - really powerful individuals, corporations, criminal organizations. They do this to influence the results; after all they have stakes in how the decision making go, and in real warfare would have an affect on success for failure. But also there is certain amount of thrill seeking. In addition, there is high stakes betting going on, over and above the stakes of the race itself.
In terms of keeping the secret - lots of powerful people with motivation to keep the secret, and to kill anyone who tries to talk. Undoubtedly there are plenty of rumors out there, just no evidence.