At present, that is. If the cost of clean water gets high enough then it becomes economical for both desalination plants to create profit but also so that civil construction projects make financial and resource security sense.

Small nuclear reactors probably would be perfect for the job. It would also be a good way to improve security around desalination plants in the future.

Although, I think if water security really did begin to get pressing then I would imagine that the government would step in to set up reservoirs and water networks to bring water to where it's needed over a much bigger distance. The US relies on their river networks because (like nearly every country in the world) settlements were built on rivers and next to easily potable water. If climate conditions change and lakes and rivers dry up then I've no doubt that public works (and money) would be fired up to build new infrastructure. With the added benefit that construction jobs would be created.

The main cost here is the energy cost. It costs a lot of money to power these plants. A secondary concern is what do you do with the various salts that you remove? Dump them back in the water? That would increase the local salinity levels and increase the energy costs needed to remove ever more saltier water.

Solar power could help, but the sheer amount of solar panels and batteries (to power during night) would make it rather expensive - as well as increase the local heat island effect.

Nothing, unless an amnesty or general pardon is given then they stay in prison. People are serving the sentences that were handed down by the courts at the time, that isn't going to change for anyone except for those whose trials are yet to start.