That is a really unhappy turn of events.  

But I want to say, while feeling devastated is perfectly natural, you have done nothing here to feel ashamed of. You weren't spending beyond your means, you certainly weren't intending to defraud any of the parties involved.  You just fell victim to the twitchiness of banks and lenders about new accounts, and ended up with a perfect storm of two of them getting twitchy at once.

Have no shame about that, and recognize that even if this unfortunate event has undone some of the hard work you have done, you still have built an excellent credit profile and some great cards other than AmEx to show for it.

If I had to guess, I suspect the real reason the CU froze the payment is because they had pulled his credit when he joined with them and knew he did not have an American Express. So a giant payment to American Express is going to look a bit suspicious. Still, it sucks that AmEx closed you down even with a thorough explanation of the situation.

Did AmEx at least allow you to pay the balance before they closed the account, or are you saying it was an instant chargeoff? How much damage is done to your scores is going to be tied heavily to the final disposition of the account(s).