As I understand it, if the US Govt chose to entirely eradicate bitcoin worldwide the most likely way would be through a 51% attack (please correct me if some other software-based attack might be more effective).

I think this makes a few questions extremely important to understand:

Would the US Govt be able to keep a 51% attack secret or shift blame to another entity? 
Would there be signals of a 51% attack or could this occur with no warning?
How much would a 51% attack cost to implement roughly?
Most importantly - Could any responses be developed within the bitcoin framework or do you see another existing or proposed cryptocurrency that would be safe from a 51% attack? (for example I hear something about bitcoins being traded "individually"? after such an attack).

Sorry I know it is a messy bunch of questions, but perhaps also the most important to consider long-term.  Please just start with the premise that such an attack would occur rather than the entirely different question of "if" the govt would ever do such a bad thing.  Thank you.