It is sort of an investment that you just sit on and then cash out when you want to.
You can literally sit on money too
I guess in many developing countries people use dollars as a safe haven and save them under the mattress. On the other hand, the bulk of the US national debt is not in dollars as many erroneously think but in the US Treasuries, i.e. government bonds. It is only calculated in dollars. And these bonds are basically as liquid as the dollar itself, so they can be effectively considered as a type of money, not a property in the strict sense of the word, something which you can use directly, for example, live in the house you bought
Exactly. It's a currency because it meets all of the requirements of a currency. Whether governments approve it as a currency or not is irrelevant. If you can break something down, it has value, and many people use it to exchange for goods / services then you have yourself some money!