  Let $(M, \varphi) = (M_1, \varphi_1) \ast (M_2, \varphi_2)$ be the free
product of any $\sigma$-finite von Neumann algebras endowed with any faithful
normal states. We show that whenever $Q \subset M$ is a von Neumann subalgebra
with separable predual such that both $Q$ and $Q \cap M_1$ are the ranges of
faithful normal conditional expectations and such that both the intersection $Q
\cap M_1$ and the central sequence algebra $Q' \cap M^\omega$ are diffuse (e.g.
$Q$ is amenable), then $Q$ must sit inside $M_1$. This result generalizes the
previous results of the first named author in [Ho14] and moreover completely
settles the questions of maximal amenability and maximal property Gamma of the
inclusion $M_1 \subset M$ in arbitrary free product von Neumann algebras.
