In F, you can count on the window shades being closed. The cabin will most likely be as dark as if it's night. While pax may keep their shades open if they wish, of course (much discussion about that issue in another thread), they will be outliers. Meals are exactly what you want when you want. Since the cabin will likely be dark, there is no "best dark seat" as such. 2A and 2K are the seats which are furthest away from other seats.

Advice about when to sleep? Depends entirely on you and your body rhythm, body clock. Personally, I'd skip the breakfast on the SYD-HKG leg. Breakfast on the plane will never be as good as the breakfast which The Peninsula puts on in The Wing. That applies to CX F service, too; so I'd eat breakfast in The Wing. The F cabin on 257 won't go dark immediately after takeoff; but let the cabin crew know that you plan to sleep, so that they are at least aware of your preference. They can, and do, serve meals in the dark. When pax are eating, they simply use their reading and overhead lights. And, of course, there's always the off chance that you'll be the only F pax, in which case the cabin will be dark when you want it that way. It's happened to me twice, both on 884, HKG-LAX, 744, old F.