1) That depends whether the power seemingly must by its nature be unitary.
2) This has never come up. It’s an interesting question. See “1.”
3) Militiamen at the time of the Constitution’s ratification were state soldiers. There had been state regular-army units during the Revolution, but having those units, militia units, and congressional units proved unworkable, and so the power to raise regular army units was delegated to Congress.
