Any wood flooring over concrete in contact with the earth usually requires the services of an experienced professional to get it right the first time. Whether or not you're going to install plywood first (an obsolete method in my professional opinion), the concrete needs to be FLAT and you need to make it flat before starting the job. Wood manufacturers will state how flat that needs to be. And, you need to know how much moisture is coming through a slab before you do anything to it or apply any flooring to it.

I think it would look great, especially with a sealer that brings out the color. How much does this slab weight? I had a 40 by 60 inch slab cut out of my 10 inch thick foundation for a basement egress window. I thought about using the slab as part of the retaining wall in the new window well, but when I figured out that the piece was gonna weigh about 2000 pounds, and that the concrete cutting company wasn't gonna help me move it, I just dug a deep hole and dropped it in.

It's not that big of an area and that all definitely has to go and the ground graded properly. You still would want a pad at the very least for the lower door so I think a overall new pad/patio with the footers set properly for the deck posts would be the way to go. It will give you potential storage under the deck as well.

You're asking, can you add a 3" layer of footing on top of a 3" layer of footing that is already in place ...? Can we presume the finished footing is also wide enough? (It should stick out beyond the outer edges of the brick wall by about 6" each side. If it doesn't, add width to the whole footing in the second layer.) I say that that it will be okay. I wouldn't bother boring holes and adding vertical rebar between the layers. (Sheering of the layers is most unlikely. It's only going to matter if you have an earthquake big enough to destroy your house!) But 2 or 3 strings of 1/2" rebar in the footing running lengthwise would be a good idea. (It would aid against cracking but If the footing were to crack, it would keep the wall sections aligned.) Freshly wash the first layer free of all dirt before pouring the second layer, so that there is a good bond between them, and pour while the first layer is still damp (not wet.)