Henry,
I work with Delta and cannot tell you how carefull to be using a buddy pass to Europe in the summer. July is better than June, but you are pretty close to 4th of July. I recommend leaving Tues/Wed and returning Tues/Wed/Thurs. These generally give you the best chances.

As far as departure city, just stick to JFK. JFK "generally" has less non-revs than Atlanta and CVG. I would not rule out CDG, but generally its not the greatest as I see you have read in my post long ago.

What I would do is make a list of all the JFK options and then narrow them down. To check that many flights on TraveLine is insane so I have a better recommendation. Check this website out: Delta Seatmaps. It will allow you to see the number of seat assignments for flights. This is NOT anything more than a way to see what flights may have seats. A flight may show here lots of open seats, and then you call TraveLine to check actual availability and you will find that its full. Thats because people may have tickets but not actually have seat assignments. Like I said though...It will narrown down the possibilites for you.

As far as the eurail idea goes, another idea is Easyjet. Easyjet is an extremely low cost carrier where you can travel all over europe for cheap, especially if you book now. Many people find now that planning your trip on Easyjet can be cheaper and much faster than Eurail train passes. See www.easyjet.com

Also be ware that hotels fill up fast and on weekends. Research the hotel and hostel options before you get here. Hotels are not cheap like they used to be in Europe. Hostels are a better option for the budget traveller, but have a reservation for weekends at least 1-2 weeks in advance.

Happy travels and let me know if you have anymore questions.

Matt