I want to jump on it it early so I have the best chance of getting a ticket for our trip, but it seems to me that there is no way to book the outgoing flight if the return flight availability is not yet known, is this correct?
That is correct. You'll either just have to wait till the return becomes available or book a dummy return and then pay a $100 change fee when the return flights become available, the latter being a risky move since return availability may not become available.