If you are plucking the bird, make sure you also scald the feet at the same time. 140-150F for 30-60 seconds should do it. The skin on the feet should pull off the feet just like socks. Then the nails should pop off the tips and the feet will be clean on the inside.
An empty crop is much easier to deal with than a full crop the size of a baseball when it comes to cleaning. That said, I always withold feed for the 12 hours over night. So I just take it out when they go to roost, and butcher in the AM. That way they aren't "starving" as much as they haven't...
Usually 12 hours is enough time to clear their system. Just make sure they still have water available so things keep moving.
Since these are old layers, best choice is going to be slow cook, soups, and pressure cooker. It will help to brine/soak in buttermilk/age the birds for a few days...