Howdy and welcome!
On #1, yes the are under the baffle plate is considered cook chamber volume.
On #2, we have our own calculator we use. it is a little different than the feldon one. ours is located at smokerbuilder.com/pitcalc.
but as far as the height goes, as long as the throat opening is located at the top you are ok. the downside is that would make the cook chamber door opening possibly on the high side. the baffle plate needs to be above the top of the throat opening and the throat near the top. if you add 3" to the distance between the BP and the lowest CC Rack then the bottom of the door would be almost 36" if the bottom of the firebox sits on the ground.
Cheers for the quick answer, will get onto the smokerbuilder calcs and figure it out. Am a tall guy so CC can be a little higher than normal, but yeah, still don't want a rack at shoulder height!