First - if the production stopped when they began free-ranging, I agree that the eggs are most likely somewhere. There are lots of stories about people finding piles of eggs in wierd places, so that's your best bet. And the post about predators finding them first is a good point.
Second, I also agree checking for mites is a good idea, based on your comments about the BR's vent.
Lastly, who says "cull" means "kill"?! You could easily sell 2.5-yr-old hens. Be honest about their laying, but someone looking for a ready-made flock on craigslist would undoubtedly jump on it. Especially if they know the whole story. I really believe they are still laying...somewhere.