1. That is to the almost-sole purview of the Commander-in-chief, or anyone smart enough to hardwire their way through a PALS and arm one (in which case shit has to be bad enough that such a breech in security would go unpunished).

2. America signed a 'no-first-strike' policy. Meaning we'd need to have probable cause to believe that someone is fielding non-conventional weapons. Whether these weapons are directed against other Americans or anyone in general is up for open interpretation (but in short, simply using them is usually grounds for invasion).

3. Even so, we utilize a counter-force concept of Nuclear Utilization Targeting Strategy. So quite likely a limited response in-kind. An example is developing/deploying MOAB shortly after the WTC collapse.