Confused... Pasty butt or more???

Ok, at first I thought it was just pasty butts, so I took to soaking rear ends in Luke warm water... Chicks still dying. I've lost around 95% of the chicks I've hatched. They seem fine one minute dead the next literally within hours. I've been told by the owner of our local co-op that it sounds like coccidiosis given the chicks go from healthy to lethargic, unsteady, flu-ish (some have been sneezing with runny eyes etc) then dead. Most have also had the poop build up on and around their vents. One I cleaned off very gently and as soon as I set him back down he fell over, stretched out and died. The co-op owner recommend an antibiotic other than the duramycin I tried before that's specifically intended for coccidiosis so I'm trying that. It's only been 24hrs but we have only lost 1 more, the other that was doing badly hasn't gotten better but no worse either. All my babies are kept in the incubator for approx 24 hours so they are 100% steady dry and warm before moving to our brooder in my bathroom under the heat lamp, I also have a heater in there for extra warmth given the below freezing temperatures at night. The bedding was pretty dirty for a few days due to a financial emergency but has been kept clean and dry, changing water every day, I drain the available water and put the remaining back in every few hours, fresh feed every day. I'm keeping all the new hatchlings in a separate unused brooder to prevent further infection. I'm so lost! It more started with eggs that were shipped, I made sure to only order from certified flocks.

Do you know what the temperature is in the bathroom? A heat lamp alone should be plenty and they should have the option to get to an area that is a bit cooler if they so choose.
My understanding of pasty butt is that it happens when chicks are dehydrated. I know you said you give them fresh water everyday, but if it's too warm for them they could still get dehydrated anyway.

I can't speak to whether or not it's coccidiosis because I have not had to deal with that yet. You are right to keep the new chicks separate though. I would also try supporting the sick ones with probiotics such as yogurt and apple cider vinegar.

It usually runs around 70-75 degrees ferenheit in the bathroom. I have the lamp on one side, the water is on the cooler side of the brooder. I'll turn the heater down and see if that helps. Thank you so much!

No problem, good luck!