Hi Nate ; Good luck, I think you will be happy with the results.
I have not heard of anyone burning dry dung, but folks have been burning it for century's so I'm sure you can add some. Might make extra ash though ? Big deal if it does , scoop it out and carry on.
When you go to building your mass , do not use the sand, use lots of rock with clay if you have any or just plain dirt.
Quick question, are 55 gal barrels plentiful, and cheaply available ? If the answer is yes then you might consider building a 1/2 barrel bench rather than a piped bench. Quicker , less cob, faster heat, the down side is it will cool off sooner than a solid mass.