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.
You are correct. Although technically, dissolution is not a reversible process. That's why you need to input energy to reverse it.
Carnot cycle, technically, doesn't apply to all energy sources directly.
For example, solar panels have their "hot side" at around 6000K, so Carnot efficiency would be close to 100%. Real solar panels have other limiting factors, and I believe the absolute achievable theoretical maximum is around 80%.
On the other side of the spectrum, wind turbines have very lousy Carnot efficiency because they're exploiting a temperature difference of just a few degrees. However, the "Carnot tax" is not paid by us directly, so we don't really care about it.
