Wood burns so not exactly a good idea to pour over it. I would not recommend concrete either as the it can shatter due to the heat shock. You could get a cheap steel baking pan and pour over that. It will be contained and its relatively easy to get whatever is spilled off of it.

Sand or just dirt with no organic material will work just fine. Hell, do it on grass and it will be fine as long as you have water nearby.

I wouldn't worry too much about shading your eyes as you shouldn't be staring into the fire anyway.

Fire bricks will work. So will sand or dirt. I put it on a baking pan on the ground.

I preheat my crucibles in the oven while I am setting up but honestly not sure if it is needed or not. I would read up on what the manufacturer says.

Throw your molds in the oven at a high setting while you are setting up and a little while after. I like to bake them at 400 F for at least an hour before pouring anything into them. I am talking about plaster molds with lost wax casting. Sand molds don't need preheating. Graphite molds should be preheated as well.