Nope did not read that one and not going to get into reasons why aftermarket feature sets don't get used all the time in OEM systems. Lets just say car companies are almost always 2-3 years behind the aftermarket which can move faster to changing technologies. That said things like sub control and alike are often seen as potential issues for oems as it induces a potential consumer produced quality control problem due to abuse. Bass and treble controls can be limited in eq and limiters to reduce the potential failure, but level controls are not frequency variables.

Feeding the 4 channel input using front and rear is really not an issue as long as the front end is feeding stereo into both. If the head unit uses any type of surround processing or virtual surround then there is a significant issue as the rear channels are typically limited bandwidth, only have a portion of the full signal ( what is uncommon between l/r channels) and phase shifted to create a sense of space. Essentially when you fade you reduce the input level to one pair or another. If I remember correctly the rear input pair is feeding channels 3-6 which are configured to run the subs behind the seat. Doing so does not produce a sub gain level control it as it can only work to the negative by under driving the input relative to the front as one fades. It does not increase gain above the standard settings