Since you likely need to dig down to level the the ground and to make room for your gravel base, you can fill it with the soil. And if the hole is still there, you can fill it with gravel. Ensure the hole is packed solid as well.
Yes, I recommend wearing safety glasses, gloves, and a mask with ventilation. I also recommend wearing long jeans so you don't get anything on your skin.
Purchase a vice drill, which is a pencil-shaped vice with small drill bits. They are available in craft stores. You may need to purchase smaller drill bits if the hole...
My best guess is that the pebbles and brick contain limestone, which is easily dissolved in water and will raise the pH of the soil. You'll have to water with...
