1. The code is open, and no exploit has been found.
2. The consensus model would require involving a large number of people over a global geographic region in the deception.
3. Regulation and market manipulation are both risks present in every market (including bitcoin) but bitcoin eliminates several manipulation vectors (such as inflation).
4. If the government dumps a bunch of bitcoin it might cause a lull in bitcoin prices, which is likely to drive a bunch of purchases, and the price will stabilize. If anything, it might incentivize more people to get into bitcoin as it would present an opportunity for return on investment.