  The ability to manipulate entanglement between multiple spatially-separated
qubits is essential for quantum information processing. Although
nitrogen-vacancy (NV) centers in diamond provide a promising qubit platform,
developing scalable two-qubit gates remains a well-known challenge. To this
end, magnon-mediated entanglement proposals have attracted attention due to
their long-range spin-coherent propagation. Optimal device geometries and gate
protocols of such schemes, however, have yet to be determined. Here we predict
strong long-distance ($>\mu$m) NV-NV coupling via magnon modes with
cooperativities exceeding unity in ferromagnetic bar and waveguide structures.
Moreover, we explore and compare on-resonant transduction and off-resonant
virtual-magnon exchange protocols, and discuss their suitability for generating
or manipulating entangled states at low temperatures ($T\lesssim 150$ mK) under
realistic experimental conditions. This work will guide future experiments that
aim to entangle spin qubits in solids with magnon excitations.
