Christian's body and coffin didn't end up on the plane simply because it reflects the story that happened in Island-timeline where his body is missing (albeit not the coffin). It seems that everything that happened on the Island is happening in LAX-timeline too, so maybe it was just to highlight the way the characters will ultimately end up in the same places - ie in their relationships with each other.
Jack's dad's body ended up on the sunken island during the flight when the plane shook.
The coffin was never loaded onto the plane. It is LOST in Australia.
Though none of these statements can't be proven (yet), it seems likely the coffin fell off the plane and that the opening camera, falling straight down into the water, is actually seen from the coffins angle.
It's pretty unlikely that the cargo area on the plane would randomly open in mid flight, and one (or two) lone pieces of cargo would go flying out without anybody becoming aware of it. More likely that the luggage was loaded onto the wrong plane in Australia.
Perhaps the original fall into the water was from the coffin's angle, but after that, I don't think the coffin would have been cruising around the ruins of the island like it was one of those motion-movie rides. It probably would have just landed.
The knives also disappeared, the knives are crucial because they kill Jacob and Christian Sheppard is crucial too to the island (his importance still only speculation), so maybe they are constants in the two dimensions
Maybe this is related to that episode where Richards asks child Locke "wich object already belongs to him", and John answers the knife
but richard alpert wouldnt understand that! awesome
In keeping with the theme of everything being more positive, Jack's dad's body arrives in LA too late for the funeral. Jack gets to reconcile himself with his father's memory and death, and uses the opportunity to change the shoes on his father before he is actually buried.
The coffin is later found and returned to L.A. The reason for the coffin being misplaced is so that Jack and Locke can have a chance encounter.
When the plane flew over the Island, the Island took the coffin and Locke's knives. In a way, the island reclaimed everything that had been on it previously except for the passengers.
Wouldn't that mean that the island took ALL of the cargo?
Is it possible in the original timeline that smokey somehow hid Christains body so it wouldnt get discovered, and he could freely use Christains image without suspicion?
The body not appearing in the coffin showed a failure of the HOPE of the survivors. The premise was that time would reset, all would be well, story over. Well, that didn't happen. It's an indication that the Losties can't simply ignore their destiny. Destiny will find a way and it all starts with Jack and his funeral and will spiral downward from there. It's the starting point of the correction. Up until that point, everything had gone to plan, Charlie defied death, Locke got his walkabout, Hurley was happy. The lost coffin was the first real indication, at the end of the episode no less, that this plan wasn't going to work.
Exactly, the body went missing in both timelines. This is a similarity not a difference. You can't change fate, that's been a reccurring theme in the show already with Charlie's near death experiences etc. Desmond was also on the plane so he still had unfinished business too. You can change the circumstances, but not the outcome.
The coffin was not on the plane because the supervisor of the baggage department refused to load it, in spite of what the customer service representative in the terminal agreed to do. It is lost because it was susequently loaded on another flight, possibly of another airline, but no one updated the database.
The contents of the coffin (the box) were decided at the moment it was opened. Think "Schroedinger's cat"...