This seems to be a tricky question. Let's see if I understood the whole thing and look at it from the inside:
1) 4 million BEX exist.
10 million BitUSD exist, each (on average) with a collateral of 0.2 BEX. The internal market prices them at 0.1 BEX. 2 million BEX are bound in collateral for BitUSD.
2) The external value of BEX drops from $10 to $6.67.
The internal price of BitUSD rises to 0.15 BEX.
3)This causes the first margin calls. (If all BitUSD had been created at the same price, all with exactly 0.2 BEX collateral this would cause an instantaneous short squeeze. But let's assume they were not.)
The first margin calls destroy some BitUSD and drive the price up to 0.16 BEX (with the external value of BEX unchanged at $6.67 one BitUSD is now $1.07).
4) $1.07 per BitUSD triggers shorts to create new BitUSD at 0.16 BEX, these now have a collateral of 0.32 BEX.
5) This goes on for a while: The exteral price of BEX continues to drop, more margin calls kill old BitUSD with small collateral, more shorts create new BitUSD with large collateral. Finally 3,999,999 BEX are bound in collateral but still 10 million BitUSD exist. The exteral value of BEX is near $5.
6) The last, only available BEX is highly sought after because it is the only way to short BitUSD. Its price should not be able to fall below $5. Demand should stop its external price from falling this low in the first place.
The external price of BEX can only fall more if more free BEX is available. The only way this would happen if BitUSD were voluntarily sold internally.
--> If all longs hold onto their BitUSD this should maintain their value.
Not the total amount of BitUSD is limited by the exchange rate of BTS/USD but the exchange rate of BTS/USD is limited by the total amount of BitUSD.