Hi John....you actually were right about the "prehistoric skeletons" part. DE is made up of mined deposits of diatoms, which are microalgae.
https://en.wikipedia.org/wiki/Diatom Their cell-walls, like many plants, are made in part of silica (which is why grass is so hard on teeth of grazing animals). Other salt- and fresh-water crawling things have calcium carbonate bodies (clams, crabs, etc).
Interestingly (perhaps...it was to me), diatoms are one way they figure out if someone died before they were in water or after. Diatoms inside the lungs mean the person probably drowned and inhaled water. If they died or were killed before they got put in water, no diatoms -- except for in sugarmakers who don't wear dust masks perhaps.
The amount of permeate to run through the press to flush it out depends on the size of the press. Just start running permeate through and measure the sugar content with a refractometer. When it gets down below the sugar level you want (we target about 1%), then stop capturing it. Ours is set so that once we finish filtering a batch, we connect to the hot permeate line and use the pump to run the permeate press washing right back into the sap tank. We have a meter in line so we know how much liquid runs though, but you can do the same with a timer. No more carrying liquid around -- getting too old for that. The only place we have left where we use buckets is the hood condensate, but that'll change this year to make it a little easier.