Naked Crab, Missing Limbs

[TheDivineZero]

My aunt has been out of town, and I have been tasked with caring for my younger cousin's hermit crabs. I came over this evening to find a naked crab upside down under a piece of cuttlebone.
I quickly setup a EE and sand mixture mixed with salt water in a tupperware, put fresh and salt water dishes cut out of the bottom of some solo cups, and a little food dish with pure honey and natural applesauce in as well. I rinsed a shell in salt water and left it nearby. he has not moved since he has been found, and we did find a leg an inch away from him, but I did not recognize any other damage to his abdomen. A shell was also nowhere near him.

A note about his tank: He is rough in the care department. When he comes home, I am going to help him improve his tank.

Here is the care sheet filled out for the best of my ability about his tank.

. What kind of substrate is used in your tank and how deep is it?
He uses EE, reptile sand, and by the food dishes, some calci sand. It is about 2 1/2 inches deep.

2. Do you have gauges in the tank to measure temperature and humidity? If so, where are they located and what temperature and humidity do they usually read?
No, he does not.

3. Is a heat source used in the tank? If so, what?
No heat source.

4. What types of water are available (fresh or salt) and how is the water treated (what brands of dechlorinator or salt mix and what ratio is used to mix it)?
Fresh and salt. They use well water, no dechlor. For salt, he has Instant Ocean.

5. What kinds of food do you feed and how often is it replaced?
He has All Living Things Variety Mix, and a powder treat. Every other day.

6. How long have you had the crab and what species is it, if known?
Unsure, he is a PP.

7. Has your crab molted, and how long ago did it happen?
Doesn't appear to have had a recent molt.

8. What type of housing are the crabs kept in, what size is it and what kind of lid is on the housing?
20 gal, wire lid covered witha towel.

9. How many crabs are in the tank and about how large are they?
4, 1 inch and less is my guess, as I have seen only 2 of them.

10. How many extra shells are usually kept in the tank, if any?
There are about 8-9.

11. Have there been any fumes or chemicals near the crabitat recently?
No.

12. How often do you clean the tank and how?
Unsure.

13. Are sponges used in the water dish? If so, how are they cleaned?
No.

14. Has anything new been added to your crabitat recently?
No.

15. Is there any other information you would like to share that might be helpful (anything that is regularly part of your crab care, playtime, bathing, etc.)?
The other day, I saw small mite-like creatures in the food dish, in which after I changed out the food, and my dad, who moved him to his iso said he saw some.

16. Please describe the emergency situation in detail.
What I did not add above, I used an extra wire tank lid, and covered most of it with a damp paper towel. I did ask my family if he smelled, and my sister said and I quote "He smelled kind of like the sea, but not really super fishy or rotten."

What more can I do for him?

I will try to add pictures shortly, but I have no access to a computer right now. Apologies for mistakes, as this was written on mobile.
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Edit: I am planning on cleaning out the 10 gallon I have left from my previous crabs and moving it to their house, as it has heating pads attached to the side. I also have an air pump with a stone I'd like to add as well. What I would like to do is add more sand/EE in the tank, and nestle the Tupperware in the corner, as I don't want to move him any more. I am probably going to put a divider in and fill a small section by the heating pads, as I don't have enough Sand / EE on hand. Other than that, last night he did not move at all. When we had lightly sprayed him after putting him in ISO, he did not respond. Tonight I am also bringing over some calcium powder to put by/ mix with honey. I don't want to bother and try to feeding him, just in case it might just be a nasty surface molt. Is there anything more I can do?

[KayedeeLove<3]

If he is alive and naked he needs to be re-shelled asap! Clean out the shell (close to his size) real good and gently slide him in. It may be to late for him, it sounds like a shell fight if you can't find his shell. Or he could have dropped it from weakness do to conditions. Im sorry! Good luck!

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[TheDivineZero]

I'm honestly not sure if he's alive (it doesn't look very good), I'm just maintaining conditions incase he is... Unfortunately, I do think it was a shell fight, given that the crabs are all close in size, and all but one turbo is taken, otherwise, it's painted shells. I'm just trying my best at this point. Thank you c:

[TheDivineZero]

I have another question... we have been checking for the 'death smell' but I'm unsure... they say he smells fishy, and I'm beginning to think he is gone Due to some anxieties I have, I just really can't bring myself to smell him... But if I keep getting answers like I am, I may have to make myself do it. He just looks very dry and limp. Humidity is up, but no improvement. When I first found him up to now, he abdomen has looked shriveled up, but I don't think that indicates anything.

I investigated further in the tank today, and found his smaller pincher and one of his entire back legs, as in the leg most to the back they use to walk. So, he took a good beating and this isn't even accounting for other damage I haven't picked up on.

My cousin and his family come home tomorrow or Monday, and Tuesday I will go over and I will have a talk with him about his tank. I hope I get it across to him this time. I've sent him this site, and the care guides several times and he has changed nothing He had 3/4 of a bag of EE, a nearly full bag of sand, and after bugging him often to add sub, no dice. I'm going to be picking up a 75 gallon, so I think that if he picks up his game, and actually listens, I will bring my empty 55 gallon and we can set it up together.