Greece will default in mid July without external aid

I don't think usa will default since it is mostly borrowed in us dollars, which it can print in unlimited quantities. Inflation - sure, it's possible; default - highly unlikely.

Greece is in this situation partly because it cannot print euros by itself; in fact this is what the opposition argues for: don't force the population to pay for the country's loan which are euro denominated, which would move greece outside of eurozone and blow its currency to something almost useless, but something which they'll be able to print.