At what point does something like this become a national security issue?
I know these modern titans of industry answer to no one, but perhaps they should?
It might as well be true, but then you should also expect other countries to start dealing with Apple as a part of the US gov. and not a purely private party.
That is exactly what happened and that's why they approached Tim Cook.
Right now Apple is an international consumer company at its core, which makes its positioning a lot different than defense contractors or power producers for instance.
Also, yes, Lockheed Martin does operate in different countries (Ireland included) and most likely does negotiate quite heavily with factories around the world.
Or a threat. An entity with a trillion dollars in reserves and devices in billions of people's hands, including hundreds of thousands, if not millions, of government employees. That's a lot of leverage.
What do you mean? They certainly answer to China.
National security issue, though? The US government has no desire to call Apple on security or privacy. Doing so would be a pure disadvantage for them.