Yes, this is an international flight. ATL-SIN. For a domestic flight I won't even bother asking for anything as it might be solved by an upgrade and sitting in a middle seat is not the end of the world on a flight of a couple of hours.

We all know some agents are very helpful, some are not. I have two Transpac trips coming up. I don't want to ask for something that's not Kosher, i.e., come across as DYKWIA.
Boeing 777-LR has eighteen bulkhead seats.
I would like to have a bulkhead seat assigned so that I can put my CPAP out of the way of other passengers. It's a full flight, only 16 seats, as shown on the seating charts are available for selection, in additions to the ones that DL releases to airport control/GA on the day of departure. It's ideal if there is seat vacant next to me then I don't have to put my CPAP on the floor, but that's something that's not likley to happen on this flight.

I understand if a person in a wheel chair needs a bulkhead seat, an occupant of a bulkhead seat can be asked to move. It's unlikely that all nine bulkhead seats would be needed on a flight, although it could happen. It's equally likley or unlikely nine elites, or even fewer DMs/PMs/GMs or SMs with companions, would be travelling with infants, or require that the airline "must" provide them with bulkhead seating in economy comfort section. Bulkhead seats for parents of an infant are also first come first served. I don't know who trumps if a pax with an infant and a CPAP user, with equal status ask for it at the same time. There is another bulkhead further back for those unwilling to pay for a seat in economy comfort.

Would DL ask an elite to vacate an Economy comfort seat for a non-elite pax with an infant who has not picked and paid for an economy comfort seat?

Even on a TATL flight, I coould manage without my CPAP and hence a seat appropriate for a CPAP. ATL-SIN is twice as long as a TATL.