Why? You knew that the pending charge would expire. There was no reason to do anything unless the charge posted, at which point you could have disputed it because it was processed for the wrong amount.

I would write a concise, polite, apologetic letter to American Express explaining that you panicked when you saw a large pending charge on your account, called, explained it poorly, and created confusion resulting in the closure of your account. Ask that they reinstate your account. I would not include disparaging characterizations of any employee, nor mention your own merchant account.