Bumping this thread to say I am having the same experience: SAS-ticketed transatlantic codeshare between NY and CPH and neither airline using either PNR will let me pick a seat on the DL-operated segment on a Main (N)/Go Pro fare. Each airline blames the other sigh. I'll try OP's solution to contact Delta again at T-5 but just a heads up for others booking SAS-plated trips on DL metal, you'll have issues picking seats and that may be a problem for you if seat choice is important.