It's actually not sustainable even with that (also, as far as I know the government isn't taking money from social security). Social security isn't like a bank account where you pay in until you're 65 and then live off your saved money (though that is the case for similar programs overseas,like Australia). Social security works by taking taking tax revenue from people currently working, and then directly transferring it to retirees. This addresses the issue that Australia's superannuation system has where inflation eats into the value of the saved funds, at the much more severe cost of requiring a precise balance of workers and retirees, a balance that is out of wack.
There's not enough money for that:
We spend far more on welfare programs then the military, and even substantial cuts to the military won't close the gap. We also probably can't afford those cuts, we have existing obligations that must be covered, and the navy is one of them. The US navy is the only navy powerful enough to counter China's growing naval ambitions, we can't cut back on that.
Air Force, Space Force, and Coast Guard, actually. The Marines are technically part of the Navy, they're not a totally independent branch.
However, that's the same overly literal understanding of the Constitution that leads to the "it says welfare, therefore the government can do anything" understanding of the constitution. It's self evident that at the time, thec intent behind that clause was to allow the federal government to fund a full military if it was required. Likewise, we know what the 10th amendment was understood to mean at the time of ratification.
Even if the law was 100% clear there would still need to be a SC for other circumstances. The SC would still have to be the final arbiter of appeals, address disputes between court circuits, resolve disputes between states, resolve cases involving foreigners, etc. As laid out in the Constitution, article III, Section 2.
Article II, section 2 and 3 vest the president as the chief diplomat for the United States, he therefore has authority do whatever is required to fill that role, unless you're prepared to argue that he can't, say, hold a formal state dinner because the constitution doesn't include guidelines on table settings?
As for receiving gifts, you're again wrong. The Constitution says officals of the United States cannot receive gifts
without the consent of congress, it does
not say that they cannot receive gifts period.