  The $B_s \to D_s^{(*)\pm} K^\mp$ decays allow a theoretically clean
determination of $\phi_s+\gamma$, where $\phi_s$ is the $B^0_s$-$\bar B^0_s$
mixing phase and $\gamma$ the usual angle of the unitarity triangle. A sizable
$B_s$ decay width difference $\Delta\Gamma_s$ was recently established, which
leads to subtleties in analyses of the $B_s \to D_s^{(*)\pm} K^\mp$ branching
ratios but also offers new "untagged" observables, which do not require a
distinction between initially present $B^0_s$ or $\bar B^0_s$ mesons. We
clarify these effects and address recent measurements of the ratio of the
$B_s\to D_s^\pm K^\mp$, $B_s\to D_s^\pm\pi^\mp$ branching ratios. In
anticipation of future LHCb analyses, we apply the SU(3) flavour symmetry of
strong interactions to convert the $B$-factory data for $B_d\to
D^{(*)\pm}\pi^\mp$, $B_d\to D_s^{\pm}\pi^\mp$ decays into predictions of the
$B_s \to D_s^{(*)\pm} K^\mp$ observables, and discuss strategies for the
extraction of $\phi_s+\gamma$, with a special focus on untagged observables and
the resolution of discrete ambiguities. Using our theoretical predictions as a
guideline, we make simulations to estimate experimental sensitivities, and
extrapolate to the end of the planned LHCb upgrade. We find that the interplay
between the untagged observables, which are accessible thanks to the sizable
$\Delta\Gamma_s$, and the mixing-induced CP asymmetries, which require tagging,
will play the key role for the experimental determination of $\phi_s+\gamma$.
