I love the show, but they did say stuff like "no one comes back from the dead" or "no time travel" which they ended up contradicting.
no one ever came back from the dead. there were instances of ghosts and the afterlife but no one ever came back. One of the big rules was "dead is dead." They also never said no time travel only that "what happened happened" (another repeated phrase) and in fact the time travel was like a predestination paradox, whatever they did in the past fit in with what already happened and had no effect on the timeline. even when they tried to change the timeline, they couldn't.
They never said they had everything completely laid out, but you gotta admit they liked to make it seem like they did know everything. That's JJ Abrams' modus operandi, and when he quit the show the left Lindelof and Cuse to sort out his mess.
I don't think they were left with a "mess," they were left with a rough plan and then worked out how to make it a reality. Things only really cemented after year 3 when they inked out a deal with ABC to end the show in year 6.
