As far as zero-sum goes, with the rake [the fee per pot casinos charge] it is negative sum. Further, when ESPN pays Caesars to broadcast the World Series of Poker, none of that money is added to the prize pool, so it is not like professional golf where sponsors help add to the prizepool [or maybe entirely add to it, im not sure]. Some individual players are sponsored so for them, they are "freerolling" [meaning they cant lose money, they can only win as somebody else is paying their way].
For your creativity question, it depends a lot on how players think. There are a lot of small situations that occur in a game where what I have been studying previously makes me think of a better play while I am actually playing, but most of that is based on concepts I have been previously discussing and thinking about. However, Phil Galfond, one of the top players in the world, has said that he learns the most by playing and thinking deeply about every situation and figuring things out on the fly. He is clearly an exception to the rule, and I he is where he is at now because of all the hours he played leading to now as well as how much he discussed the game with other very high level players.
For your creativity question, it depends a lot on how players think. There are a lot of small situations that occur in a game where what I have been studying previously makes me think of a better play while I am actually playing, but most of that is based on concepts I have been previously discussing and thinking about. However, Phil Galfond, one of the top players in the world, has said that he learns the most by playing and thinking deeply about every situation and figuring things out on the fly. He is clearly an exception to the rule, and I he is where he is at now because of all the hours he played leading to now as well as how much he discussed the game with other very high level players.