We give two generalized Moreau-Rockafellar-type
results for the sum of a convex function with the composition of
convex functions in separated locally convex spaces. Then we
equivalently characterize the stable strong duality for composed
convex optimization problems through two new regularity
conditions, which also guarantee two formulae of the
subdifferential of the mentioned sum of functions. We also treat
some special cases, rediscovering older results in the literature.
A discussion on the topological assumptions for the vector function used in the composition closes the paper.