What "additivity property" are you talking about? Do you mean perhaps the fact that <u, w>+ <v, w>= <u+ v, w>? That's really a property of the inner product itself, not of the space.
I'm reviewing Meirovitch's "Methods of Analytical Dynamics," and I don't understand the commutation of the derivative from r to dr:
$$
\mathbf{F} \cdot d\mathbf{r} = m \ddot{\mathbf{r}} \cdot d\mathbf{r} = m\mathbf{\dot{r}} \cdot d\mathbf{\dot{r}}
$$