DL could care less about the miles / segments and how they are credited, it's simply that it draws an administrative line in the sand about reversing credits and issuing debit memos to other carriers.

Only way to get this done is if you can show a provable error on DL's part and you can't do that here.

Not particularly an odd question. It happens all the time and DL is far from alone among carriers in refusing to do it.