I would fill the bog with a mix of sand and peat moss..
I would go 1/3 sand to 2/3 peat, by volume..
(just because I dont like 50/50..I think its too much sand..but opinions vary)
then a top dressing layer of LFS across the top, once the plants are planted, to prevent the peat from spashing around in the rain..
As for drainage..I would drill drainage holes 3 inches down from the "surface" of the bog..
this keeps the bog very wet overall, but not waterlogged all the way to the top..
I have found over the years with my minibogs:
http://gold.mylargescale.com/scottychaos/CP/page5b.html
that I dont need to know or care exactly where the water level is..
I have never wanted or needed any kind of "water level monitoring" system..IMO its completely unnecessary..
eventually you learn to just go by feel..if the bog seems dry, add some water until water comes out the drainage hole..then let the water level drop again for awhile..there is no need to keep it constantly "topped off"..with a bog that big, you can probably go for weeks between waterings..
(and IMO a fluctuating water level is "natural" anyway)
I wouldnt bother with rocks in the lowest recessed section..just fill the whole thing right up with sand and peat..
Scot