Some cameras don't WB well, even in AWB in conditions a camera should WB sometimes things don't work out right. This is a way to correct that. In the end I try to get the colors semi accurate and pleasing to my eye. In Lightroom I do the same with temp and tint for quick corrections. If I need some real fine tuning in LR I would work with the color channels or go to Photoshop and do the individual RGB curves. You can do that in Davinnci as well, but I find you can get the same result in scopes. Its all working with the same data, just in different user interfaces.
That is conceptually incorrect. You don't look at RGB channel to evaluate white balance. If your in camera is wrong you white balance in post and trust your eyes if you don't find something to balance. A frame that is white balanced will not have RGB parade at the same value. A grey slate has all values of RGB parade the same
So your idea of using the RGB parade to white balance only works if you filmed a grey slate full screen you balanced that particular frame and applied