Assistant US Attorneys.

yes

There is no exact figure on the total number of prosecutors employed in the U.S. as it varies across federal, state, and local jurisdictions. However, it is estimated that there are tens of thousands of prosecutors working in various legal offices and agencies throughout the country.

Prosecutors can choose to Nolle Prosse a criminal charge but they do not have the power or authority to dismiss a charge. Only judges can dismiss a charge. I seriously doubt that any statistics are kept on the number of state AND federal charges nationwide that are Nolle Prosse'd.

The overall 'boss' of a state prosecutor is the State's Attorney General. The prosecutors office is part of the Executive Branch of government - not the Judicial Branch.