If the producers pick the category everybody gets, then naturally people who can get the producers to stick them with their category of choice
I would also like to mention that Keelan could've gotten knocked out if Keelan was chosen by the randomizer at any point during the finale, as he would have to play something other than periodic table(We saw this with Mena in the beginning). The randomizer didn't pick him, and it once again illustrates the issue everyone was saying(if you are the last to get picked, you will win) and now it's just being gamed by winning one duel, waiting for everyone else to go, keeping your OG category, hoping the leader picks off the others, and once again, hoping you didn't get picked by the randomizer. If Keelan gets picked at any point during this finale by the randomizer he almost for sure does not win as he likely sucks at Shakespeare. Catherine(blue jacket) tried to knock Keelan out via "Russian roulette" as she called it, and failed. It's once again about being picked as the last one, but this season it was just about being the last person to have your OG category.
There is something inherently wrong with the randomizer that needs to be fixed because the two-square thing makes it so easily gameable until the end of the season. It's better to be randomized then to be challenged because you will keep your original category in the process. You are then safe with your original category until someone challenges you(not to mention the edge/corner strategy) if you go back to the floor because your original category scares them off, which in turn would keep you safe until everyone has dueled. But all this power, for one duel(yes you might have bad options for duel choices) seems a bit too OP.
All categories were played, and we never found out officially, but I think a contestant said that it was going to be international Foods(the unplayed category from season 1).