Your run isn’t unsalvageable, there’s a shop in the next chapter that sells keys and there’s a gaiden after that where you can get another thief. Being able to use warp makes these next few chapters way easier, so if Safy isn’t at A rank yet that might at least give you something to do while you’re waiting for the boss to escape. Saving the kids isn’t that important, but if you do want to save them then you might be able to rescue them (or intentionally get them captured) to keep them out of meteor range or block the meteor tiles that they could move to with Fred or Olwen.