I hate to say it, but that's what XTS is there for. To keep BitUSD stable, XTS must suffer the whims of the market. From an economic perspective, there really is no choice but #3. And hopefully, the market is robust enough and the technology is strong enough that things will keep on moving without this sort of issue. It gives me more faith in BitShares every time I see a post like this, though. You are thinking through these issues, testing, and seeking community consensus. This is going to be a damn fine product.