WhatThe,
What do you want to see from the Rose and Bernard story? What do you think they can add? I'm not understanding the passion to see them return...
As far as shooting the flashbacks from the beginning, in a perfect world, I am sure they would have done that, but in reality, they are dealing with actors that can be egotistic, venues that can change, and other venues that won't allow for a perfect plan to take place...
See Eko... he had contract issues, but the writers would have loved to have him around as he was a valued character. But it doesn't always work out.
Enjoy the fantasy of the show, and don't concentrate so much of minor visual details... it will take away from it.