4. “How long can we continue to put off eventual repayment of our collective debts?”
From what I understand, the USG does not and cannot invest or save money, at least in any traditional way. Sure it can “invest” in R&D or infrastructure, but it does not have a vehicle in which to put money aside. The fed may insert or remove money from supply, but this is not the same as, say buying stock in IBM or a CD. There is no lockbox, the funds received are spent, and some of it is spent to service debt.
Uncle Sam can (and should) limit how much debt it takes on, because the future debt service can become manageable, but it cannot set money aside for the future, except by paying down existing debt.
The Chinese government is actually quite an active capitalist, since it owns several profitable businesses, but that is socialism, which we don’t do because we are capitalists… But thinking about that makes my head hurt. Damned commies, can’t they do anything right!