The only indication of an overbooked flight was when he was checking on July 4 and saw 17 seats and there were only 4 available for the July 23 flight
At that time they were not booked on the flight, A July overbooking would not have cause airport changes months before the flight. I suspect his flight from St Vincent had been removed from the schedule when his booking was rescheduled from St Vincent to St Lucia
If AA changed his booking month earlier due to the flight going away, the most they owe him would have been a refund. The first email he received about a change, seems to show the first outbound leg showing as from Miami. I had this issue in my BA.com booking. AA.COM showed 2 return segments, but BA.com showed the return as LHR-DFW, and not BER-LHR-DFW. If a schedule change email shows something missing, that is when you check, especially when dealing with 9 people. At some point he indicated he had received at some point with the airport code "buried" in it. I doubt it! If he travels as much as he claims, the story sound off, and the news story is too one sided to be called "news". Additionally the report of the events is a bit confusing.
I just feel AA 1427 showing 4 seats for July 23, available in his call on July 4, is NOT oversold. I suspect they were unable to get rebooked to it, because they wanted 9 seats, but at the time they had already been booked from St Lucia, so they did not have any seats on 1427 to have been IDB on July 4. I doubt AA declares an oversold situation 17 days in advance of a flight, especially when the plane has 4 available tickets. That is still time of cancellations. So, my suspicions is they were booked from St Lucia months earlier due to the St Vincent flight being removed from the schedule, and then it came back