I've elevated the issue with both AMEX and DELTA teams respectively. Both companies are aware of the issue. Delta is claiming the transfer from AMEX contained corrupted files, and AMEX is claiming Delta's system isn't accepting the miles due to an error. The teams at both companies seem to be working on it, but there is still no estimate or ETA on when the affected customers SkyMiles should arrive in their Delta accounts. Following up on the AMEX side seems more appropriate as the transfer is initiated with them. I will be following up weekly with AMEX until they've posted. (This was a big statement for me, unfortunately, 8,000 Miles are at stake.)