In any event, DL could make the sale and the ticket would still hold for as long as 72 hours for the downstream partners to confirm. If they do not, the reservation is cancelled.

Your sole recourse here is to ask DL, as you have already done, to reach out to the onwards carriers and ask them to confirm positive space. That may take a few days. The answer will likely be no and that is the end of the issue. It is your problem because DL cannot require another carrier to open (or create) space.

It has been this way for 50+ years and is as frustrating today as then. Just more self-evident online.