Motional ground-state cooling and quantum-coherent manipulation of mesoscopic mechanical systems are crucial goals in both fundamental physics and applied science. We demonstrate that the motional ground state can be achieved in the highly unresolved sideband regime, through coherent auxiliary cavity interferences. We further illustrate coherent strong Rabi coupling between indirectly coupled and individually optimized mechanical resonators and optical cavities through effective dark-mode

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... ction. The proposed approach provides a platform for quantum manipulation of mesoscopic mechanical devices beyond the resolved sideband limit.