Snakes eat eggs and keets, so you might want to snake proof the area your Hens will be laying if you confine them. I agree with Colby, you need to trap or kill the opossum. It knows where it can get a free meal every night and it will keep coming back until you stop it.
If your birds roost in the hen house at night, then close them in each night and don't let them out the next day until you collect their eggs. A lot of us with Guineas have worked with our birds since they were keets so that they are trained to come running for treats and food when they are called, and especially so they come in at night into a safe predator proof coop and/or covered pen. If you haven't been working on this with your birds and they don't roost in the hen house at night, obviously it's going to be difficult for you to get them in once you let them out. You could try taking their food away around noontime, then let them out after you've collected their eggs and then before dark try calling and coaxing them in with food. They may have a hard time roosting in the hen house if you have clipped their wings.
One more thing to keep in mind is that when you all of a sudden completely lock birds up that are used to free ranging all the time (or stress them out by clipping their wings), they may not breed as much/as often as they did before, so your egg fertility may go down... (which is a big consideration if you are collecting their eggs for incubation rather than consumption).
Sounds like you have your work cut out for you in order to be able to collect eggs. Best of luck, whichever route you choose.
