It doesn't have to be a week. Just a couple of days so dates for flight 16 would appear. Then I could change flights 15 and 16.. and a few days later change flight 16 again to the date I actually want.

It's basically the same issue everyone faced when standard awards could not be booked one way. By the time the dates for your return flight became available, the dates for the outbound segment were gone. But in that case you could simply hold the outbound flight and then add the return segment a few days later. However, since this is a RTW, I don't think the agent would be able to swap/merge with flights from a separate PNR.