I have an AP, you book up to 7 days out without a reservation. With a reservation, you get the 60 day window. We haven't stayed past 7 days with the AP so not sure how that works if I had a longer stay. I'm not sure how an AP changes anything. But perhaps I'm misunderstanding your question.
From what I've been told, throwaways are actually paid for, not cancelled, but perhaps I'm wrong on that and it's common to book and cancel.
The original question was:
"could someone who only plans on vacationing for three days book a ten day trip, open their window on day sixty (which is really days 68-70 for the final three days), wait ten days, and then cancel the first seven days of their trip, essentially booking FPs for the three days they really want to travel 70, rather than 60, days in advance?"
In order to be able to book 10 days out- you have to have a 10 day pass. If I'm wrong on that , I'm sure someone will let me know, but fp is tied to your ticket, not your reservation. Your reservation only unlocks the 60 day window.