  Extending investigations of M\'etivier&Zumbrun in the hyperbolic case, we
treat stability of viscous shock and boundary layers for viscous perturbations
of multidimensional hyperbolic systems with characteristics of variable
multiplicity, specifically the construction of symmetrizers in the
low-frequency regime where variable multiplicity plays a role. At the same
time, we extend the boundary-layer theory to ``real'' or partially parabolic
viscosities, Neumann or mixed-type parabolic boundary conditions, and systems
with nonconservative form, in addition proving a more fundamental version of
the Zumbrun--Serre--Rousset theorem, valid for variable multiplicities,
characterizing the limiting hyperbolic system and boundary conditions as a
nonsingular limit of a reduced viscous system. The new effects of viscosity are
seen to be surprisingly subtle; in particular, viscous coupling of crossing
hyperbolic modes may induce a destabilizing effect. We illustrate the theory
with applications to magnetohydrodynamics.
