  We develop a version of Seiberg--Witten Floer cohomology/homotopy type for a
spin$^c$ 4-manifold with boundary and with an involution which reverses the
spin$^c$ structure, as well as a version of Floer cohomology/homotopy type for
oriented links with non-zero determinant. This framework generalizes the
previous work of the authors regarding Floer homotopy type for spin 3-manifolds
with involutions and for knots. Based on this Floer cohomological setting, we
prove Fr{\o}yshov-type inequalities which relate topological quantities of
4-manifolds with certain equivariant homology cobordism invariants. The
inequalities and homology cobordism invariants have applications to the
topology of unoriented surfaces, Nielsen realization problem for non-spin
4-manifolds, and non-smoothable unoriented surfaces in 4-manifolds.
