I believe that this figure is % weight on the front of the car. This seems to be opposite to reality with respect to 'less tail wag', but remember that this isn't reality, it's just a physics engine. :) The engine may be calculating the weight on the front wheels and deciding that more weight = more grip = more stable when steering (see below). The main way the engine controls steering stability is via the 'Oversteer' setting.
As for why less weight on the rear causes you to have a lower top speed, the physics engine in TDU seems to have a close relationship between 'weight' on the driving wheels and and amount of the car's power that will be 'applied' to the road. It's almost like you're wheel-spinning if you don't have enough weight on the driving wheels. Parameters like downforce, weight & weight distribution have a huge effect on applicable power. Again, this is sometimes contrary to reality.
The 'Y' axis is vertical and in my experience this setting has very little or no influence on the car. Sometimes there are parameters which are non-functional (or effectively so) and I believe that this is one of them. If you want to reduce body roll, you are best to adjust the suspension instead. AFAIK there is no 'left/right' COG adjustment (& no real reason for one either).