Hi
1. AF premium economy is a separate cabin with much better seats, slightly better food, and noise cancellation headphones on some flights, while on DL the best you can hope for is "Comfort Plus", which is a seat in one of the front rows of the main cabin with slightly increased leg room and free snacks from business class (in case they don't eat them all).
2. No idea -- I always get priority boarding because of my status. But I think that DL should comply to AF rules. To make sure just try to give them a phone call
3. Again, it's better to call
P.S.: If there is such an option, try to book via JFK and connect to AF007 operated on Airbus-380. Their PE cabin is nicer than one on A340. And the noise level in A380 is substantially lower than on "conventional" A340/B777