Did DL say you can have the J space if there is any seat, or does it have to be an award seat at a certain level or otherwise restricted inventory seat of some kind?
If they will give you the last J seat on their metal (as they should since this whole problem is their responsibility) then I'd wait and see if SQ will open anything.

You can always watch J inventory and if it starts dropping as you get close then call up Delta and get bookd on their metal. Same for SQ... if their J inventory looks wide open then keep waiting, but if not then grab the Delta seat.