City to City Buddy Pass?

I am traveling ATW backpacking, volunteering, etc. and have a buddy pass that I'd like to make the most of. I know that I can use it open-jaw because I've done it, IE: JFK> MXP and FCO>JFK. I have a couple of questions:1) Can I do something like JFK> MXP and FCO>SFO? Or do I have to return to the same airport I started from?2) If I have to return to the same airport does my starting airport have to be in the US? (Seems like a strange question, but I know there are rules like Delta flight + Delta equipment, so I want to check.)3) Are layover cities allowed? IE: JFK>CDG (a day or two?) and then onto FCO?Thanks for any help you can give! Happy travels!