If you concentrated sunlight to points in the ocean, could you make the water evaporate significantly faster to induce a substantial amount of rain? Assuming you could do that, could you use silver iodide to bias rain toward certain areas of land?
I don't think it would be a good idea. Solar power plants are effective only because they direct light beams to a small area, which result in high heat rate concentration. To make the water evaporate "significantly" faster You would need huge amount of mirrors or else the heat rate concentration would be too small to make a difference.
Answering to the second question, as far as I know, silver iodide, dry ice or liquid propane (used in methods of cloud seeding) can affect the intensity of precipitation from existing clouds or cause existing clouds to grow rapidly, but I don't think they can help forming clouds on a clear sky.
So, you use mirrors to focus sunlight onto a small area of water to increase the evaporation rate, and making the air at that point more humid. But the mirrors are deflecting the sunlight away from other areas of water. This lowers the temperature there, reduces the evaporation rate, and decreases the humidity of that air. So you have low and high humidity in masses of air on a scale of at most kilometers. How long before turbulence eliminates these variations and puts you right back where you started from?