This would all point to an alternate time line. Desmond being on the plane instead of in the hatch pushing numbers to save the world. If we were to flash to 2004, and the plane landing instead of crashing that is. What I am about to say would be considered a SPOILER so stop reading now if you are opposed to them.
Season 6 Episode one has been named LAX. Or at least that is what I have heard. Knowing Lost writers like we do, is it not possible that the X in LAX be a variable, meaning LAVariable and not the airport? So this would widen the possibilities more considerably then just, hey guess what guys, Jacks plan worked!
I love how the writers of this show think! We can misguide our viewers so easily just by the titles we give each show!
I am not saying your theory is wrong, but placing Desmond on 815 would be more then just a simple course correction wouldn't it?
I do believe in an alternate time line, though I did not use to. But I don't think the variables or differences would be that astronomical. Hawking made sure Desmond got to that island, something I think will never change, unless there is a second instance of Des, like Miles the baby, and Miles the grown man....
I have just confused the heck out of myself, heh