The problem with the O6 saving everybody:
Either they still have to fulfill their destiny and the reason for them being there has not yet been revealed, or
It's sort of a complex chicken-and-the-egg thing.
To save everyone on the island Locke had to turn the donkey wheel.
To make him do that, he needed a reason to leave the island.
Bringing the O6 back provided that reason.
So at this point it might be inconsequential that he actually succeeded.
Or it could actually be both.
I'm still a bit unclear on why the donkey wheel was off-axis when Locke turned it. Hadn't they been transported back to ancient times (when the statue was still around) when Locke turned it? So wouldn't that have put them many years before the donkey wheel would have been put off axis in the first place?
Or does the chamber containing the donkey wheel exist "outside of time" or something?