Can you clarify what you mean by this? Just to clarify, once the flights are released (330 days out), you can book if there's inventory in the award class. Once you booked, each change will cost more points and potentially more money, and that's assuming there's inventory in the new dates you want.

Unless you want to see SEA, why go there when your cruise starts in YVR?

Also, I don't think there's a direct flight from Washington to Edinburgh, let alone First Class. You'll have to go through London, which is why I suggested earlier that you London to Edinburgh by train.

Here's what I suggest, from my knowledge of F routes. No doubt others can improve on it.

SYD-LAX (QF)
LAX-DFW-YVR (AA)
YVR-JFK (CX - Very difficult to get 2 F award seats)
JFK-DCA (AA or make your own way)
IAD-LHR (BA)
LHR-EDI-LHR (BA, or make your own way)
LHR-IST (BA)
IST-AMM-DXB (RJ, you can go back to LHR on BA and to DXB in F, but it'll take much longer and more expensive, LHR taxes are high)
DXB-HKG (CX, no F here, I don't think, and only regional J, which can be quite poor for such a long flight)
HKG-SIN (CX, very few F here)
SIN-SYD (QF or BA)

This is the most direct routing, but be flexible in order to get to the places you want.