If I'm reading this correctly, the first brightness setter call will be ineffective until a second later, when the brightness value gets applied or any later values that might have been set in the meantime.
No, when the first set brightness call arrives it starts a timer. Every next call to set brightness will restart timer and set new brightness to m_brightness. After the last call to set brightness will arrive, timer will wait one second (and if will not be restored again) then will call the worker which is set the needed brightness (m_brightness).