I think that for $859 I'd probably fork out the money and earn the points on LY. Since I don't fly west from Israel I'd see flying DL as a waste of my money/points. BTW have you figured in the ATL-MIA cost? I'd rather burn the points when the fare is more like $1100 r/t. Earning the 1400 trips on LY flights is the equivalent of 7 r/t to the east coast or far east in Y. Also, check out the sticky Earning LY Matmid points with Israeli CCs for a pretty thorough discussion of what it takes to earn the 1400 points on CCs.

As for the summer. This past year I had absolutely no problem using my LY points to fly 4 people to EWR July 2 with 2 people returning mid July and 2 three days before school started. That said, I booked my award seats in January and had them ticketed (points deducted) 2 weeks beforehand. I was able to move the flights around a day or two up to 2-3 weeks ahead of time. Those tickets would have cost me $1200 each at that time of the year when you include the fuel surcharges and the like.