murray I will attempt to answer a few of these questions.
1) The DSB in question is not lighted, so therefore it is a bit different than one you would find in a display tank. I would bet that the anoxic layer is somewhat shallower than in a display tank where the bed is lighted and churned by the livestock.
2) Surface area certainly is helpful. However in this case the entire surface area will be usefull (not covered by rock and corals). Again this is a bit different than the DSB in a sump... but I do understand the question and don't have a better answer.
3) Partially correct for the aragonite to disolve your PH would have to be low enough to kill everything in the tank... OR WOULD IT? Down in the deeper layers of the bed, the PH can be substantially lower. This is where the disolution takes place. As the disolved aragonite and water from deep in the bed make their way to the surface they mix with the main mass of tank water..... so yes you do get your buffering
Hope that gets you started.
Bean