I use a stainless steel mortar. The oyster shell I buy is pretty thin and soft, so it crushes into smaller pieces easily. I don't see any reason to separate your egg and oyster shells. The birds will pick out what they like.
I would remove a male or two to get closer to a 1:5 ratio. Other than that, especially with winter hatches, sometimes it can take up to 12 weeks before they lay.