  Establishing the conditions under which orbital, spin and lattice-pseudospin
degrees of freedom are mutually coupled in realistic nonequilibrium conditions
is a major goal in the emergent field of graphene spintronics. Here, we use
linear-response theory to obtain a unified microscopic description of spin
dynamics and coupled spin-charge transport in graphene with an
interface-induced Bychkov-Rashba effect. Our method makes use of an SO(5)
extension of the familiar inverse-diffuson approach to obtain a quantum kinetic
equation for the single-particle density matrix that treats spin and pseudospin
on equal footing and is valid for arbitrary external perturbations. As an
application of the formalism, we derive a complete set of drift-diffusion
equations for proximitized graphene with scalar impurities in the presence of
electric and spin-injection fields which vary slowly in space and time. Our
approach is amenable to a wide variety of generalizations, including the study
of coupled spin-charge dynamics in layered materials with strong spin-valley
coupling and spin-orbit torques in van der Waals heterostructures.
