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.