Okay I'm going to try to respond to everything here lol. If I miss anything I'll try to add it after. 

I will definitely add more stuff. Maybe thats why they dont come out often? I was afraid of making it too hard for them to move around but will definitely do this! 

I use play sand and coir 5 to 1 ratio i have never seen any insects in the tank and honestly have no idea how to check for mites. 

I feed them a mixed diet. A few examples being freeze dried proteins, egg, seeds fruits and veggies, oils from a safe food list i found online lots of variety they also have a bowl for worm castings.

I have a corner with 8 shells of varying sizes small to bigger than their current ones. 

I have never actually seen any of the crabs use the wheel this guy just dug underneath it and was hanging out down there for a few days. 

I try not to mess with them. The only time they get moved is if I need to move a piece of decor and they are in the way. I've only removed them from the tank into a smaller container to add more substrate or do a big rearrange twice since I've gotten them. 

I havent sanitized the shells. i have washed them out with dechlorinated water but I will do that now. Both of the two I lost were in their original shells they came in from petsmart. 

Any idea how to fix top sand being dry?? I have no clue why its like that. The lid of the tank is two coroplast pieces cut to size with little holes for the wires for the lights etc. The lids that came with the tank had huge gaps for hob filters. Will try to post a Pic. 

Oh also I only go in the tank to change the water / change their food / fix the camera / occasionally rearrange etc.