The sun has surface area of 6 x 10^18 m^2. Earth has surface area of 5 x 10^14 m^2. Earth can not be bombarded by more than the ratio of these to numbers with solar radiation using only mirrors. <<1 percent.

I see what you mean, but it seems like a self-correcting problem. Focus enough of the sun's energy on Earth, and it will be shrouded with a rapidly expanding ball of hot plasma, at which point its surface area would be much greater. Now you can focus even more of the sun's energy on it without conjuring Maxwell's demons.