score:7
You have two options here, and I strongly suggest starting with the first one.
Reach out to Delta Skymiles with a written request. Provide all the evidence, such as:
And send it to Skymiles support desk. From my personal experience - and I've been flying with Delta a lot - they always honored their promises. It is very likely the agent screwed it up, but since it was Delta agent, it is their fault.