  Turning a pristine Mott insulator into a correlated metal by chemical doping
is a common procedure in strongly correlated materials physics, e.g. underlying
the phenomenology of high-$T_c$ cuprates. The ruthenate bilayer compound
Ca$_3$Ru$_2$O$_7$ is a prominent example of a reversed case, namely a
correlated metal at stoichiometry that realizes a transition into an insulating
state via Ti doping. We here investigate this puzzling metal-insulator
transition (MIT) by first-principles many-body theory and elucidate a
challenging interplay between electronic correlations and symmetry breakings on
the Ru sublattice. While average effects on the Ca$_3$Ru$_2$O$_7$ crystal
structure are still relevant, key to the MIT is the cooperation of electronic
correlations with the spatial inhomogeneity in the defect regime. Together they
give rise to the emergence of site-selective Mott criticality and competing
orbital-ordering tendencies.
