I see your concern. The mirrors could be deliberately manufactured to have enough imperfections, or slight reverse curvature, to prevent this. It's possible with properly-designed optics to prevent the mirror from being able to focus on a spot smaller than, say, 1/50000 the mirror's area on the Earth's surface. Make each single mirror only optically capable of focusing down to a 3-mile diameter at best. Then it would be optically impossible to focus all the mirrors to a single 60ft spot, and the theoretical highest brightness at any point on Earth would just be regular daytime levels of light.