Correct me if I am wrong as I am not a physicist. I see a point that is important to consider, that you have potentially overlooked. First, you assume that dissolution of salt is a completely reversible thermodynamic process, which is fine. But considering it a reversible process, in order to reverse the process we need to do a certain amount of work which you have calculated. In order to do work we need an engine. The most efficient possible engine is a Carnot engine. It is known that a Carnot engine can never be 100% efficient (unless we can achieve infinite or zero temperature). Given that you calculated the amount of work needed to reverse the process, you still need to bound the efficiency by the efficiency of a Carnot engine. Alternatively you need to factor in the efficiency of a Carnot engine to get the minimum required energy input.