 Solution

For a fixed second argument the norm formula makes $P$ continuous in its first argument. Rational homogeneity therefore extends to real homogeneity by rational approximation; additivity and $P(iu,v)=iP(u,v)$ then give full complex linearity in that argument.

The real pairing $B$ is symmetric, and $B(v,iu)=B(iu,v)=-B(u,iv)$. Hence $P(v,u)=\overline{P(u,v)}$, giving conjugate linearity in the second argument. Finally $B(u,u)=Q(u)$ and $B(u,iu)=0$, because $\Vert (1+i)u\Vert =\Vert (1-i)u\Vert$. Thereforewhich is positive for $u\ne 0$. This proves $P$ is a complex inner product inducing exactly the original norm. The assumed Banach completeness makes the resulting inner product space Hilbert. This is the Jordan-von Neumann theorem.