Ok a few things to go over, I'll do them by number of the thing you filled out.
1. The recommended amount of substrate is 3 times your tallest crab or 6 inches, whichever is taller. 2-4 inches isn't enough for your crabs to be able to molt safely.
2/3. 76 is on the low end of what is safe for pps. Ideally your heat and humidity should both be around 80. I have never used a strip thermometer but I'm guessing it wont give you an accurate reading unless it's inside the tank it's self. Someone chime in if that's incorrect. Once you move your heaters and thermometer I think your heat will rise and if not you'll be able to tell so you can fix it. I think I read that you already moved the heaters to the backsides of your tanks above the substrate lines?
4. Both salt and fresh need to offered at all times. The all living things conditioner isn't that great. There are other brands sold for fish which are more efficient and better for your crabs. Also, marine salt (like for fish tanks as well) such as instant ocean is also better for your crabs. Other salts lack the nutrients that crabs need to remain healthy.
5. Check your commercial pellets to the safe foods list. Most are not good for crabs. Fresh foods are great and most of the time you can share whatever you're having for dinner with the crabs. I usually save a piece of meat, fruit, or veg on the side and cook it plain (no seasoning or salt) and put it in their fresh food dish. I'll link the fresh foods list and another thread on foods in a few minutes.
6/7/8. Six crabs is the max I would put in a 20g. The absolute minimum tank recommended to keep pps in is 10 gallons. The 2.5 is going to get very cramped very fast. I would maybe look on Craigslist for a larger tank where you could keep all of your crabs and maybe use your 20g for an iso. You can usually find good deals on tanks there.
9. It's recommended to have about 2-5 shells per crab. They need some slightly small, roughly the same size and slightly bigger offered. I have 6 crabs in my tank right now and about 20-30shells for them to look at. I have (over time) collected extras so that I can change them up and offer different shells every once in a while.
12. I don't use sponges just because of the maintenance required to keep them clean of bacteria build ups. In the future you may wanna consider using moss pits as an alternative? I think they require less cleaning, provide great boosts in humidity and my crabs love the moss pits. Regardless of anything else in my tank, the moss pit gets used every single day by one crab or more. But it does sound like you're cleaning your sponges correctly (if someone disagrees please chime in, I could be wrong).
14. I don't know if this has any validity but I use paper towels to clean my glass. It's important not to use soap and I've always worried that any soap residue/laundry detergent will effect my crabs. I have no clue if it's true, maybe just my over protectiveness coming out lol.
15. It's very likely stress. If I remember reading correctly she's in a 2.5g right? When conditions aren't optimal crabs can very easily become stressed and can suffer from pps which can kill them. I guess a fight is also possible since it may be a bit crowded in there but I think stress is more likely. She definitely needs to be left alone. No more taking her out, picking her up. She needs sometime to destress and possibly gear up to molt. I'd offer her an iso with a hit or hide for darkness, fresh and salt water, and food like protein, calcium and something high energy like organic honey, peanut butter, or bee pollen. And give her some time. Picking her up to check on her will only add to her stress. If she passes you will know from the smell.
About the backgrounds, I have no clue where to find one but I do agree that a mark down from 1,000 to 15 is something to be weary of.
Under tank heaters are meant for reptiles. They usually do go under the tank in those cases because reptiles don't require near as much substrate as crabs do. The UTH can actually hear their tanks from the bottom up, not the case with crabs. It's kinda crazy but most of the stuff offered for hermit crabs is not good for crabs so over the years crabbers have come up with other things that work better for crabs. Just most aren't actually designed for them lol.
Hope all of this helps, let us know if you have any other questions we can help you out with

and please give us updates on willow!
Sent from my iPhone using Tapatalk
Edit to add: these are the safe food links
viewtopic.php?t=92557#p852660
viewtopic.php?t=92554#p852657
This is the commercial food link so you can compare yours to it.
viewtopic.php?t=92555#p852658