Yes, the curve breaks with VC papers due to blending emulsions. Therefore, VC papers are difficult to do studies with when you are learning. Graded papers are clearer and easier to understand.

Kodak Research used a density of 1.0 to represent the mid scale. The paper plant used 0.8 but subtracted out Dmin first, so the values were about the same actually. Then the toe was measured at 0.5 and 0.3 Log E left of the mid tone and the shoulder was measured to the right of the mid tone. I think that value was 0.5 offhand but without a cheat sheet, I can't remember. The actual values were adjusted based on the shape of the toe and shoulder.

The softer the toe, the more detail in highlights and the softer the shoulder the more detail in shadows, but there are limits before things go "mushy". By definition, the eye should see things at 1.0, but it is an integrating device. Therefore it sees the entire curve when you view a photo. So, the integral should be close to 1.0. HOWEVER --- (always a joker in the deck!) most people prefer high contrast to low contrast, especially with color, and so this must be factored in. Also, lens flare lowers the contrast.