This makes no sense whatsoever. I'm guessing you have no real clue about either concept or the real issue at hand.
When you drive through a curve, whether downhill, uphill or on a plain, the determining factor of successfully negotiating that turn is the available 'grip' (I'm keeping it simple here so as not to confuse you any further) of your tires on the road. When you are braking or accelerating you are using a portion of that 'grip' that will then not be available for dealing with lateral forces (the 'curve' part of things). In fact, to maximise corner speed, you should not be either braking or accelerating at that point, i.e. you should be 'coasting'. This is why, in general terms, racing drivers get their braking done before the curve and start accelerating after the curve. (Of course they try to brake as much as possible into the curve as they try to accelerate out of the curve, but this is far beyond any level of driving skill you and most people will ever experience.)
Every automobile has an inherent polar moment of inertia. Understeer and oversteer are directly related. The vehicle is engineered to handle best with torque from the engine at the drive wheels. Without that torque, the handling characteristics change drastically. If you don’t have the auto in gear, and need to slow down (common on the downhill), you’ll be using the brakes more than you would otherwise. That will definitely affect handling. Otherwise, I pretty much agree with you said in your second paragraph.