(2) No nation can assume that it will only face one enemy at a time. Imagine the US finding itself in a war against China, Russia, India, Japan, Indonesia, and Vietnam all at once? It's hard to imagine a situation where that type of alliance would form - but you have to plan for the unimaginable to at least some extent.
(3) The US does buy some military hardware from other nations, but there are massive advantages to building your own and making sure that you retain the capability to build your own. Even if, hypothetically for example, Britain has developed a warship that would fill an American need admirably, by building its own, America, first of all, ensures that it continues to develop skills and technologies that can be used in future ships (which may not be ones they can get elsewhere) and secondly allows for the potential of export sales to other nations (such as my own country of Australia). And even if another friendly nation is building an equivalent vessel, the competition involved in building your own will tend to make both nations better at what they are doing (not to mention the potential sharing of information) - which is better? The UK and the US (and, for that matter, I would include Australia when it comes to some classes of warship) each building their own high quality vessels, each trying to produce something better than the others, and therefore each improving their own production - or just building one class that, though very good, could have been better with a bit of friendly rivalry and more people working on and sharing information with other friendly nations. And this is without considering the fact that more designs means more innovation and somewhere else to go if a flaw develops.
One obvious point I forgot to mention (still shook up by our little earthquake here). The ideal way to fight a war is not to have to fight it - a large military is far more of a deterrent than a smaller one. Countries are less likely to go to war against the United States with its current military than a smaller one. And that’s a whole other layer of protection. And not just for the US - it helps to shield a lot of US allies too.
Si vis pacem, para bellum - if you want peace, prepare for war.