If you explain you are doing the JFK thing for miles (why else would you do this), they will most likely understand and let you pass. From time to time you will get some person who just does not like the looks of your face and will send you to secondary and there is nothing you can do but smile and reexplain everything all over again.

The turn around in JFK should be very easy but you will need to clear US INS/customs and recheck back onto the outbound flight. When you arrive into Canada(at 2AM), all passengers heading to the US need to preclear US INS and customs and their counters will be shut down for the night. You will have two choices: 1)clear Canadian customs and hang out at the airport until you can check in for your DFW flight or 2)when you get off the plane, hand out in the long overhead walk//paths until the counter for US transfers opens up. I would not recommend this since there are no seats up there and security might come through and ask you what the heck you think you are doing.