When I flew BOS-FCO-BOS on Alitalia two months ago I was unable to select seats before check-in on; this was the case on both the outbound and return. The explanation from AZ was that they could not manage seat selection on the DL codeshare flight, even though it was AZ operated. Delta said they could not convert my reservation from the DL codeshare to the AZ flight without rebooking and incurring a change fee. I booked in Premium Economy, so it was not as big a problem as getting a decent day-of-departure seat assignment in economy. The PE seating on the flight was 2-3-2. As it happened, both flights had empty seats in PE, so it wasn’t a problem, but I would have been quite unhappy if I’d ended up with one of the three middle seats in PE.