Really doesn't matter what the BP says or how DL got to where it got. Treating this as a plain & simple DG, DL owes OP the difference between what he paid and the fare for what he sat. Here that's easily calculated as the miles paid for the UG.

OP's best bet for something above and beyond a refund of the miles is to send in a webform complaint setting forth in 3-4 short and concise sentences what happened and asking for a refund of the miles and some reasonable but unspecified gesture for what happened.

This is not the kind of thing which gets resolved on the phone and if it does, all it gets OP is the miles.