I have never booked a Delta FF ticket, only AA and US Air (may she rest in peace), but it's pretty easy to see whether you can book a one way. Just look at the airline's website and click 'one way' or 'roundtrip', and book his ticket separately. That would be the answer on that. And as far as miles to book going up and down, if you really are set on first class, there are VERY FEW of those on any flight. I would suggest if you have to have first class, to book ASAP. Again, on AA, the amounts to fly a certain route go up and down ALL the time, as we often take particular routings on AA that are not our preferred routing or preferred date, and wait for tickets to come available for the same amount of miles that we booked. In fact, on our last trip on AA, seats for our preferred flights went down in miles just a couple of days before our flight, so we were able to switch flights, but we were in 'cheap seats', not first class. But again, on AA, you can change FF tickets for NO FEE, unlike most other airlines, who charge a change fee.

Short answer, first class is in HIGH demand and there are very few seats and they cannot make more FC seats on the plane. Book as soon as you know you want to go!