The OP needs an itinerary that respects the relevant MCTs. Since the ticket is booked with NW miles, NW is the point of contact. NW can't force DL to open seats at the PerkSaver level, but it can find a routing on its own planes, irrespective of award availability. That should be the request of NW.

If NW doesn't serve the destination conveniently from PHL, or at all, this could get messy, as in PHL-DTW-LGA.