  We perform a consistent analysis of the alternative left-right symmetric
model emerging from $E_6$ grand unification. We include a large set of
theoretical and experimental constraints, with a particular emphasis on dark
matter observables and collider signals. We show that the exotic neutrino
inherent to this class of models, the scotino, is a viable candidate for dark
matter satisfying relic density and direct detection constraints. This has
strong implications on the scotino mass restricting it to lie in a narrow
window, as well as on the spectrum of Higgs bosons, rendering it predictable,
with a few light scalar, pseudoscalar and charged states. Moreover, we also
show that the extra charged $W'$ gauge boson can be light, and investigate the
most promising signals at the future high-luminosity upgrade of the LHC. Our
findings show that the most optimistic cosmologically-favoured scenarios should
be observable at $5\sigma$, whilst others could leave visible hints provided
the background is under good control at the systematical level.
