First, welcome (somewhat belatedly).

Second, you weren't scammed. You yourself pointed out that American Express said it was a computer glitch.

I can well understand the lack of an appropriate response from American Express based simply on the title of your post.

That having been said, you can contact American Express's executive offices and, presuming your documentation is in order, someone can work with both Delta and American Express to resolve this.

Speaking from experience, it will take communication between both companies to fix a missing MQM problem but it can be done.

It will also require a lot of patience and diplomacy on your part and you will need to approach it rather differently than "scammed by American Express."