Okay if Naomi is lying (but why?) and Cooper says the plane crashed in the pacific. Could it be that the plane takes off, disappears, plenty of time goes by and everyone has to assume it crashed somewhere in the pacific? I mean if a plane is last known to be flying over the pacific and months go by, everyone would be pretty positive that it crashed in the ocean. It is a BIG ocean, if they looked for weeks or months and never found anything. Enough time passes and any logical person would draw that conclusion. But I tend to believe that Naomi honestly believes that the plane was found 4 miles down. And chances are Cooper said Pacific because he is bad at geography or it shows just how little he cared about Locke, can't even remember where his son's plane went down. The multiverse theory...so the plane took off and went through a xerox type portal that made 3 planes and all of them crashed...yeah, no, I don't like that theory personally.
Either: A-Someone sunk a plane to fool everyone into thinking all aboard were dead. Why? I dunno
B-Someone misinformed Naomi, doubt it
C-There were 2 flight 815s, one was a set up meant to bring our Losties to the island, the other was the official flight with totally different people on it, but, it crashed too...why?
D-Everyone, including Cooper, assumes 815 crashed in the ocean and Naomi is lying like a cheap rug