I've been following this thread, and I'm very sorry for your loss
You have a beautiful heart for saving a helpless life and doing everything you can to keep your chicks happy. I do not believe your chicks have anything physically wrong with them, but I do believe that they've been exposed to lethal levels of drafts and cold.
Chicks have no capacity to regulate their own temperature until they get their feathers. Which is why they have a mother to sit on them and share her body-heat in normal circumstances. Now that the weather has gotten colder, your room is full of deadly drafts of cold air, even when you have the heat-lamp on them.
I would suggest blocking airflow on all of the sides by wrapping their cage in a towel (I once used duct tape and a tarp in my poorer days). Be careful to leave the top open for air circulation however, and for the heat of hte lamp to shine down from the top. This will keep drafts out but let air into the cage without compromising their safety.
Pasty butt is a sure sign that your chicks are chilled, it's the biggest warning mark for it. Luckily if you get heat on them soon enough, and block the drafts, they should recover.
Good luck!