There's lots of great advice here, one more thing I would add is make it clear to the CEO up front that the deadline will slip. I would make this an absolute an

There's lots of great advice here, one more thing I would add is make it clear to the CEO up front that the deadline will slip. I would make this an absolute and get the new deadline in writing, because if you use weasel words then the CEO may claim that they misunderstood. Make it clear that even this new deadline may change as your understanding of the situation develops. You should also be the one to deliver the good news about the moved deadline to the team. If the CEO is unwilling to accept this, then you should probably walk away, because it suggests that they're still in denial about what is happening.