I booked WS flights on DL miles for this summer, and Delta Diamond and Platinum members always could select free preferred seats on Westjet for them and their companion. I just checked, and the preferred seat selection (now Extended Comfort) has stuck fortunately. I believe that new WS reservations through Delta keep the same rules for Delta Platinum and Diamond members. If the Westjet elite rules are too restrictive for access to Extended Comfort seats, one can always consider trying to reach Delta Platinum status instead.