What kind of filtration system are you using? All semi-aquatic crustaceans like crabs produce ammonia as a primary waste by-product that when not properly filtered can (and will) become toxic to the crabs. When left unfiltered, this ammonia build-up leads to higher levels of nitrites and nitrates in the environment which are also toxic to the crabs. Unless your setup is properly equipped with a filtration system, this will kill your crabs. Trust me, I have learned the hard way (i.e., I have also lost many red claw crabs through this "learning process)." As you probably realize, being part of a forum such as this is very helpful to both novices and the pros.
The only reason I ask about your filtration system is because I hadn't noticed you mentioning anything about it, and before I invested in one myself I lost many crabs, and also like you, was doggedly determined to get it right, by hook or by crook.
Also consider how much food you are providing your crabs. It seems like you are sort of overdoing it. You must realize that any food that is left in the tank uneaten must be discarded (either scooped from substrate or siphoned from the water) within a 24 hour period because if it is not it will also contribute to high levels of ammonia in your tank setup, which again, is toxic to the crabs. These crabs are extremely sensitive to their immediate environment and any minor shift can cause undue stress to your little friends.
Fiddler crabs are a bit hardier and can withstand elevated ammounts of these chemicals a bit more easily and are generlly less sensitive to these types of conditions, but I am sure that someone in the forum will take issue with me making that claim. I am just speaking from my own personal experience.
Crabs!
Re: Crabs!
I concur. A filter definitely helps...but even without a filter (for more terrestrial crabs) the tank still has to cycle. I.e. the land part has to cycle so the ammonia produced by the crabs does not kill them as they walk around on it. So, it really helps to let the tank sit for a month or two, and possibly plant it with aquatic plants that come from swampy environments (like Bacopa, swords,etc.).
Re: Crabs!
I'm happy to see more people keeping crabs! I still have my Vampires and I've tried to keep several other species as well. I managed to get Bi-colors to breed, but the hatch was only 3 crablets. Unfortunately none survived. 
I also bought a pair of Panther crabs last summer. I love their coloration! at first it seemed all was going well, but as each one molted they would die. Unfortunately, they molted less than a week after arrival, so I really didn't get to observe them very much. Sometimes this hobby is sooooo frustrating... 
I also bought a pair of Panther crabs last summer. I love their coloration! at first it seemed all was going well, but as each one molted they would die. Unfortunately, they molted less than a week after arrival, so I really didn't get to observe them very much. Sometimes this hobby is sooooo frustrating... 2 red devil baby crabs
I just bought 2 red devil baby crabs, and put them in my 100 gallon tank. I did some reading and noted people having a land + water setup.. but I am wondering, what can go wrong with having no dry land, strictly water? I could do a land setup, except I have it full 98% with albino clawed frogs and a pleco already in.. I really don't want to even try to think about how to do a land setup in a full 100 gallon tank, but if my crabs will just die, under 90-100% cerrtainty without land.. I will have them moved. I'm not looking for what's best for them, because that's the difference between you having a backyard or not. You can still survive without one, of course you want one though .. (Disclaimer for any religious aquatic life fanatics) .. my tank setup is nice and the water is perfect.
Re: Crabs!
It's a bit late now...but, yes, they will die. They are mostly land crabs actually that should be kept like poison dart frogs.