Televised sewer from outside clean out and located connections from toilets in unit A and Unit D. Connection is a directional cross where paper is getting hung up at that point. Unit D has a power flush toilet. Suggest changing that to a Gerber Avalanche. If that still does not help then the floor would need to be excavated and the connections offset.
Televised ducts under slab floor. Ducts are concrete asbestos ducts with metal fittings and connections that are heavily rusted. Suggest spin cast mortaring to restore structure to the metal fittings then spray lining ducts with Duct Armor duct lining material.