Poloniex allows for 2.5x margin trading, which means the value of the margin longs, plus the value of the collateral was just under 35% of all Clam outstanding.
How are you calculating the 35%? Based on your assumptions, I'm doing: -1800 = (1939/8585)x - (2.5/3.5)x, which gives 3,685 BTC for the longs and collateral, or 43% of all CLAM.
The losses exceed the trading volume of Clam during the flash crash, so it is probably safe to assume not all positions were sold immidiately. If no Clams were sold, and their value was written down to zero, as few as 21% of Clams outsanding could have been in open positions. I would believe that Poloniex sold the open Clams positions they were unable to sell in the open market, probably at a fairly large discount.
You are right. 444 BTC of volume occurred in the first five minutes, but that only took the price down to 180K sat. There was less than 150 BTC down from there to 53K sat. That's not enough to account for a 1800 BTC loss. At most, it accounts for only 650 BTC.