Facepalm. It's staring me right in the face on that diagram. The amplifier audio circuits are grounded through the stereo. Notice there are no ground wires specifically designated for the amplifier section? The head unit has one. Just because the amp is 'connected to metal' doesn't mean that the negative wire for the speakers share the same ground path. The original head unit completely provides the audio ground for the entire system. The new head unit uses a floating ground (like most do) which isolates the entire thing from noise, but in this case, does nothing but generate tons of it. My aftermarket head unit was never designed to provide any sort of ground through the speakers (voltage would just go to an electrically isolated speaker using + and -). It wouldn't be an issue to use a floating ground in this case because the circuit is already at high power (no chance of picking up sound and amplifying it directly to a speaker).

I could go on, but...

All i have to do is pick one of the speaker (-) wires and connect it to ground on the wiring harness. The white / black wire (left rear negative) provides the cleanest ground (maybe it IS the ground in the scheme of things). Connecting this to chassis ground eliminates the entire problem. No buzz, and the slight alternator whine is gone as well.