  We classify all the $6$-dimensional unimodular Lie algebras $\mathfrak{g}$
admitting a complex structure with non-zero closed $(3,0)$-form. This gives
rise to $6$-dimensional compact homogeneous spaces $M=\Gamma\backslash G$,
where $\Gamma$ is a lattice, admitting an invariant complex structure with
holomorphically trivial canonical bundle. As an application, in the balanced
Hermitian case, we study the instanton condition for any metric connection
$\nabla^{\varepsilon,\rho}$ in the plane generated by the Levi-Civita
connection and the Gauduchon line of Hermitian connections. In the setting of
the Hull-Strominger system with connection on the tangent bundle being
Hermitian-Yang-Mills, we prove that if a compact non-K\"ahler homogeneous space
$M=\Gamma\backslash G$ admits an invariant solution with respect to some
non-flat connection $\nabla$ in the family $\nabla^{\varepsilon,\rho}$, then
$M$ is a nilmanifold with underlying Lie algebra $\mathfrak{h}_3$, a
solvmanifold with underlying algebra $\mathfrak{g}_7$, or a quotient of the
semisimple group SL(2,$\mathbb{C}$). Since it is known that the system can be
solved on these spaces, our result implies that they are the unique compact
non-K\"ahler balanced homogeneous spaces admitting such invariant solutions. As
another application, on the compact solvmanifold underlying the Nakamura
manifold, we construct solutions, on any given balanced Bott-Chern class, to
the heterotic equations of motion taking the Chern connection as (flat)
instanton.
