Residual finiteness for central pushouts

We prove that pushouts $A{\ast }_{C}B$ of residually finite-dimensional (RFD) $C^{\ast }$-algebras over central subalgebras are always residually finite-dimensional provided the fibers $A_p$ and $B_p$, $p\in \mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}C$ are RFD, recovering and generalizing results by Korchagin and Courtney-Shulman. This then allows us to prove that certain central pushouts of amenable groups have RFD group $C^{\ast }$-algebras. Along the way, we discuss the problem of when, given a central group embedding $H\le G$, the resulting $C^{\ast }$-algebra morphism is a continuous field: this is always the case for amenable $G$ but not in general.