Mountains are good water traps. Out in the West Texas deserts there are mountains with Alpine forest. There are actually Aspens in Texas, up on some of those desert mountains!

I like your biological solution to greening the Sahara.

Using an orbital lens to evaporate sea water would probably be the easiest way (after the lens has been made and put into orbit) to get water to the deserts. Or course such a tool could be used as a weapon, and in the wrong hands, that would be bad. In the right hands, that would be a great weapon.

“It’s getting hot around here,” proclaimed Saddam Insane to bin Laden.
“It sure is,” replies bin.
Suddenly ‘poof’, both get turned into piles of ash.