Nope, sorry. Your Sun Gun is permanently disabled
Because the Sun is not a point source of light, the mirror will not be able to perfectly focus its light on one spot.
Indeed, no mirror reflecting a real-world light source can achieve a greater apparent brightness than matching the temperature of its source. See conservation of etendue for the technical details of this.
The mirror as proposed by Oberth's "Sun Gun" proposal would be no more than a nice flashlight in the sky. At 9km^2 surface area , the mirror will have a diameter of just 3.4km
3.4km at 8500km altitude is a mere 0.023 degrees.
The "Sun Gun" will be able to illuminate at most 1/475 as bright as noonday sunlight.
Even if we change Oberth's design, and put the reflector in VERY low orbit (120km), it will only span 1.6 degrees, and provide at most 10.2 times solar illumination. This might, might be enough to start fires.
Remember that the light focus will rapidly diminish from this hottest center focal point, most of its light will be a diffuse spot about 5km wide, and that the mirror would be zooming past at more than 7.8km/s thus requiring very rapid adjustment of the focal point to keep one spot targeted.
But, assuming you set up a sufficiently large reflector with suitable controls:
All that is needed to "safe" it is to minutely de-focus the beam. If instead of focusing on the ground it focuses 1/10th of the distance (still an almost perfectly flat mirror), then the light intensity on the ground is less than 1/1000th the peak achievable level, and your sun gun is only a tiny bright spot in the sky, from the minute fraction of its surface that manages to reflect the sun towards you.