For the season, basically the engine select a player that perform better for FKs and another for corners, and usually performs better that anyone else for weeks.
Of course if you didn't spot him and you miss x2 fKs or corners in a single match, the ideal plan is to select another man for that task.
No, this isn't posible, there's a "assigned player" that performs better in FKs and another for corners, the empty spot have an assigned player anyway that is the GK by default, so this isn't manipulable, you can not force the game to determine X player to be the best in this field.