Oppositely charged polyelectrolytes in aqueous solution can undergo associative phase separation into a liquid-like complex coacervate phase that is polyelectrolyte-rich and an aqueous supernatant phase that is polyelectrolyte-deficient. This same complex coacervation motif can be used to drive self-assembly of block copolyelectrolytes via electrostatic interactions and can be controlled using e.g. ionic strength, pH, temperature, and polymer architecture. While there has been a large amount of research studying this self-assembly, the ability of theory to accurately capture the disparate length scales that govern the appropriate physics is limited. This is especially true when the coacervates have a high charge density; examples include biopolymers such as heparin or DNA as well as synthetic polymers such as poly(styrenesulfonate) or poly(acrylic acid). We incorporate molecular-level Monte Carlo simulations into single chain in mean field simulations, leading to a multiscale, coarse-grained description of such systems. These two length scales are connected using standard Widom insertion methods at the molecular Monte Carlo level, which provides the thermodynamic information needed to construct free energy landscapes used in the single chain in mean field calculations necessary to understand coacervate-driven self-assembly. We compare the results of our simulations to classical theories of complex coacervation and experiment. Our method demonstrates interesting behaviors in coacervate-forming diblock copolyectrolytes that reflect molecular details included, into the model, such as morphological coexistence, interfacial excess of salt, and counterintuitive salt-induced ordering.