Thanks I checked the missing part.
Your concern is reasonable. In my proposal, I attempt to deal with it by taking part of fees into the network (10% for non LTM) to prevent spamming attack.
Also, as xeroc mentioned, one possible scenario is borrow Smartcoin, keep it, and obtaining profit from fees. This seems the low-hanging fruit, however, I think it is not 100% safe method. Because this needs at least 2x collateral in BTS and the person is exposed to BTS price. For instance, if one issues 2k BitUSD with 1M BTS collateral, he can enjoy profits from fee but if BTS price decreases 10% he will lose 10% of value when his position is cleared.