I wanted to follow up my post and at least provide information about the resolution of this cancelled order, so that if anyone else had or has the same problem in future, you can see how this one turned out.
I'm usually not one for pulling and end run around the system, but after making multiple calls to customer service and being told there was nothing they could do, and with the escalation team having taken more than 60 hours to get back to me, I finally escalated it myself. On Sunday, Oct. 12 I sent an email to the address for Mr. De la vega that was posted earlier in this thread. In the email I provided my order number, as well as the names of the CS staff with whom I spoke, and the dates of those conversations.
Late in the day on Monday, October 13, I received a call from the office of the president. The person who called was very apologetic, reiterated that the error was a computer error, and they were investigating to ensure that no one else had the same problem.
She also indicated that the CS staff with whom I had spoken would be receiving re-training, as something like this should have been escalated to the office of the president long before my email to them.
Then she offered to have my wife's iPhone 6+ delivered overnight to a store here in Pennsylvania. She assured me it would be available for pickup the next day.
I thanked her but advised her that the store was two hours away, she offered to have a staff member drive it to my location so that I could receive it Tuesday. I told her that wasn't necessary, and that if she would just ship it to a closer store, that would be sufficient. That added a day to our wait, but as I indicated, I wasn't really looking to get it any faster, I just didn't want to go to the back of the line, since I'd preordered so early.
At 10:45 Wednesday, I received a call directly from the manager at the local store who advised the phone was in. I was there by 12:30, and had the phone in my wife's hands shortly thereafter.
Overall, AT&T's service, once the issue was taken to the "top" (or as close to the top as I knew how to get it) was resolved like lightning, with significant apologies at all levels for the added hassle.
Despite the order having been cancelled in error, I had the phone a little more than two days after the projected end-shipping date of 10/13. While it was frustrating for a while, the swift turnaround at the end helped resolve the frustration I was having, and I appreciate all of those folks who assisted in resolving the matter. I already followed up with a thank-you email to Mr. Delavega's address.
Hopefully, if anyone else finds themselves in a similar situation, you can get it resolved more quickly.