Desalination and Weather Impact?
Your first question, regarding salt pollution requires a back-of-an-envelope calculation. At present, about 4% of the global population relies, either wholly or in part, on desalinated water. Because this is energy intensive, it is also expensive, so such water users are parsimonious, using about 1% of the global domestic water supply. But let us, for the sake of argument, assume that the desalinated option really takes off at some time in the future, perhaps by means of less energy intensive 'forward osmosis'. We will assume that 10% of the current population (7.4 billion people) may eventually use desalinated water, yielding a daily delivery of bitterns with 2.45 x 10^^9 tonnes of salt. Compare this to the 1,5 x 10^^17 tonnes of salt already in the ocean - the salt return would be 0.0000016% of what is already in the sea. Moreover, even this trivial amount would be diluted by the hydrological cycle, because the water that is drunk, used for washing, etc, ultimately returns to the sea.

But you ask whether the local concentrated saline return would be harmful to the environment? Consider Adelaide, in South Australia, which boasts the largest desalination plant in the southern hemisphere. Very detailed studies, by both computer modelling and direct observation, show that the dispersion of the saline plume reaches concentrations undetectable against the background salinity, within a few hundred metres of the outflow. It certainly hasn't impacted the local marine ecology at all - and the South Australians are very particular about environmental issues!

Again, consider the greatest concentration of desalination plants in the world, in the Arabian Gulf. This shallow-water (&lt;50 metres) Gulf has high natural rates of evaporation, which raises the salinity from the normal 3.6% to about 4.1%. This change is orders of magnitude more than can be attributed to the salinity change from all the desalination plants combined. The ecology and biodiversity of the Gulf is still rich, and not at all affected by local bitterns effluent. Climate change will intensify evaporation, which is much more of a worry than 'desal' plants. 

As to your second question about changing the climate and increasing rainfall by pumping bitterns into the desert, there is a massive problem with that. As the salinity increases by evaporation, the vapour pressure at the lake surface decreases, which suppresses further evaporation. Since we would be dealing with hyper-saline brines to start with, we would rapidly approach an equilibrium in which the massively decreased evaporation would equal the natural deliquescence of the salt. In fact, evaporation would win, but only slightly. The loss of moisture to the atmosphere would be barely noticeable, and not sufficient to terra-form the down-wind area. 

I suggest that the impacts of global warming will massively eclipse any trivial changes brought about by evaporation basins. If you don't believe me, I suggest you get some hydrologic and geochemical textbooks, and do some mass-balance calculations of your own.