the reserve exists solely for the purpose of the fast path.
I disagree because the idle controller works on both slow path and fast path, it is constantly trying to reach the target torque reserve with the slow path and it is using the fast path to compensate for the speed of the slow path.Torque reserve gives the idle controller more leeway to respond.
I understand what you mean, but the way you say it makes it seem like the throttle angle is fixed once you reach idle speed.
It's not. It's closed loop. If you turn on e.g. AC then the ignition will momentarily advance, and then as soon as the slow path controller catches back up the ignition retards again to provide the target "reserve".