The recently described monodimensional hybrid pseudo-perovskite ((CH₃)₃SO)PbI₃ exhibits complete Pb²⁺/Bi³⁺ miscibility in the B site. Heterovalent substitution imposes that charge-compensating defects are introduced in the lattice as well. This paper reports the energetics of point defects and the interaction between charge-compensating defects that occur upon Bi³⁺ doping in ((CH₃)₃SO)PbI₃. Periodic DFT simulations were employed to analyze the formation energy of Pb²⁺ vacancies, (CH₃)₃SO⁺ vacancies, iodide vacancies, or the insertion of interstitial iodide ions. Solid solutions with a large Bi³⁺ content were modeled considering different charge compensation defects (Pb²⁺ vacancies, (CH₃)₃SO⁺ vacancies, and interstitial iodide) to consider electrical neutrality. Both the formation energy evaluation and the structural parameters after relaxation are in very good agreement with available experimental data (both X-ray diffraction and absorption spectroscopy), confirming that Pb²⁺ vacancies are indeed the primary ionic charge-compensation mechanism leading to solid solutions described by the formula ((CH₃)₃SO)₃Pb_3xBi_2(1−x)V_(1−x)I₉ (0 < x < 1), where V indicates a B site vacancy. As a consequence of the complete miscibility, the defect concentration at intermediate compositions is considerable, and it is probably responsible for various peculiar structural effects observed, e.g., in ((CH₃)₃SO)₃Pb_0.99Bi_1.34I₉, which are related to short-range order phenomena in the chain composition. Atomistic simulations were then carried out to estimate defect–defect interactions as a function of their respective positions inside the linear PbI₆/BiI₆ octahedral chains. The resulting bond energies for atom pairs and triads effectively explain the relative stability of defect clusters. This approach is expected to shed light on the short-range order in this class of doped materials, but it can be extended to other structures as well. The understanding of defect structures and control of structural and electronic properties have implications for the engineering of doped materials, which find vast applications in optoelectronic devices.