Good evening Tom.
I have personally seen this before. What an awesome build. Very practical if you have access all the way round your build.
I googled and watched this build in a slide show to see how it worked. Totally awesome!
How deep? The best way to know is to contact your local building inspector at see what a footing has to be in your area. I wouldn't provide anymore info then I had to. They could make you pull a permit that could cost a lot in the long run. Play dumb and just ask questions. It they tell you a footing has to be 2 feet, that has to be the bottom of the footing. The purpose is so when the ground freezes it won't raise the footing. Just like a house footing. In my area the footing has to be 16in. below grade.
Yes, once the foundation is poured, you can bring it to ground level with block. Be sure it fill all voids with block. Here is a picture of my fireplace and smoker foundation. I laid block off the foundation up to where I wanted to start.
The open channel is for the clean out for the fireplace and smoker.
3. Yes, I poured my footing a foot bigger then the finished build. This adds to stability. Bring the block to where you want your fireplace to start and pour or lay 4in cap block to start your build on. Bring your face brick off the footing.
Something to remember. Standard brick and block work on 8 and 16in. You can guess at it or if your set on getting it perfect, buy you a white rule that has brick work layoff on the back. A great rule even for experienced bricklayers.
Lastly, I can not stress enough, once finished let your build cure for at least 30-45 days. Then start with small fires and build up over a couple weeks to a full burn. If you build a big fire in green masonry, I promise it will crack. Better safe then sorry.
If you have any questions feel free to ask.
I'm looking forward to seeing this built. Great design for folks with wide open spaces.
Edit: This is my setup complete.