In a handful of places, it is. In the rest, it's so much more expensive than alternatives that it's borderline insanity to use it.
Like ice: ice is cheap in Antarctica! But importing Antarctic ice to a desert would be absurd.
Solar is cheap when active, and the good news this is one of the cases where intermittency doesn't really matter. We should really explore more use of intermittent power for cases like this where the limited reliability of wind and solar is less significant.
It's still significant if it means that your desalination plant is only active 50% of the time, since it means you need twice the capacity.
True. This condition only holds in situations where the energy costs dominate the physical plant costs
Fortunately, water usage is correlated with sun intensity.