If these crabs ate each other, there would be shells left over. Chances are, they escaped or burrowed, and you just didn't find them. THey are fast, they can climb, and they are smarter than you think. So a tight fitting lid is a requirement.
As far as your new set up, I really suggest a simple beach. Pile big stones along the back or one side. Fill in between with sand or gravel, and put a couple more rocks on top. Then put a shallow (1") of sand on the other side, so the rocks are the shore, and fill with about 2-3" of water, so the whole beach is 1" above water, plus the other rocks. I think this will be the best way to simply give your crab what you need and be able to watch them. I could suggest more complicated methods, like setting up a vivarium with a small pond the way you would for frogs, but this is the simplest thing you can do. The 5.5g paludarium took me several days to build, and would be unsuitable for a few reasons, primarily because it's so easy to escape from and hide in.
As far as food goes, they don't eat that much per day, but do constantly act like they're eating small stuff I can't see. I try to offer a variety of things to eat, and I see what is missing each day. I'd suggest putting some food in a tiny glass bowl and see what's left in 12 hours. Hikari Crab Cuisine, Frozen Bloodworms are what I would start with. Both are available at Petco, etc. Shrimp foods are also good, and crayfish foods. They like meat and fruit. So, you can also try a smooshed green pea, or a similar sized piece of pineapple. They also require calcium for proper shell development. So you should supplement that by dusting fruitflies or crickets with calcium supplement for frogs, and feeding those sometimes. 1 live cricket will go a long way. That's a day's food for a crab right there. Don't take it from your basement, the pesticide on it will probably kill your crab.
