Are the pump and drain considered common property among the building's residents based on the engineer's confirmation, the change in electrical connection, and the contribution to costs, or should additional measures be taken to ensure this? And is it permissible to charge the building's residents for the cost of pumping water from the apartment after it flooded from the building's garage?

Disputes can only be settled through the judiciary, because the judge is the one who undertakes to examine the details of the case and the evidence, and his ruling is binding. It is not permissible to oblige the residents of the building to bear the cost of what you paid to remove water from your house, because they did not do anything that would obligate them to guarantee, and the reasons for guarantee are three: direct deprivation (of property), causing damage, and unauthorized possession.