@jods
Apparently, I can't quote you since your post ended up in your quote, but anyway....You are probably right.
My thought was, the FP+ window will open (for you, on the 16th) and you can make FP+ for length of stay. So the number of days you can make FP+ is tied to the FD portion. Let's say you're taking a day off in the middle or your last day you have an early flight, so no parks. Now, I'm wondering if you could make a FP+ for the 15th?? You have a room reservation, so the system knows you're there. You just don't have tickets attached to that reservation. I don't know if it can be done, but I'd be trying it.