By thermodynamics, the molar volume within a phase can change because of three things (in almost all cases): changes in pressure $p$, temperature $T$, and compositions of the species $x_1,x_2,...,x_n$ comprising the phase. As you said that $p$ and $T$ are invariant, in those two processes you have the same phenomenon, namely the change of compositions.
In my opinion, it would be not wise to classify the molar volume regarding the type of process. Firstly, this would lead to a possible confusion that two different processes under the same $p,T,\{\mathbf{x}\}$ exhibit different molar volumes, but this is not true. Secondly, there are many, many physical processes and chemical processes. For example, when we go up in height, the temperature and pressure decreases, and thus molar volume experiences a change. You would have to add a subcategory there, because this is a physical process but not a dissolution. Digging even more, if you take into account electromagnetic radiation, you must add to the physical process (gravity), the radical-based mechanism that changes the constituents of the atmosphere (ozone and oxygen), and again the molar volume changes at a certain height. Likewise, you would have to add another subcategory in the chemical process, because not all homogeneous phase reactions are induced by the interaction with photons.
Thermodynamics is a fine way to skip all these exceptions, in one way. So, I would simply say:
- The first case represents a change in molar volume due to the change of the composition of the substances as the reaction progress. I would like to say that the formula you have is not true. The molar volume is in fact
\begin{equation}
V = \overline{V}_\ce{A}(p,T,\{x_j\})x_\ce{A} +
\overline{V}_\ce{B}(p,T,\{x_j\})x_\ce{B} +
\overline{V}_\ce{C}(p,T,\{x_j\})x_\ce{C} +
\overline{V}_\ce{D}(p,T,\{x_j\})x_\ce{D}
\end{equation}
where the $\overline{V}$'s are the partial molar volumes of the species. They are not constants, and in the general case they are function of pressure, temperature, and also the molar fractions of other species. Almost always they must be obtained by experimental data. They could also be calculated with statistical thermodynamics under some assumptions, and postulating a form of the Helmholtz free energy.
- The second case represents a change in molar volume also due to the change of the composition of the phase. Initially we only have solvent, i.e. $\ce{H2O}$, and after some time some solute enters the liquid phase so we have $\ce{H2O}$, $\ce{Na^+}$, and $\ce{Cl^-}$.
Both cases respond to the same explanation: change in composition $\{\mathbf{x}\}$.