I wouldn't install the heat lamp they're a fire hazard. Not a lot of point in rescuing them if you cook them alive in a fire.
I don't know how sick these poor chickens are but unless it gets extremely cold, they shouldn't need additional heat.
Decent feeding should supply the calories they need to keep warm.
Don't bother with the DE either. There is no evidence that it works. Seriously don't waste your time and money with it.
Permethrin powder, or spray for the mites and lice.
Nutri Drench or any vitamin supplement will help give them a bit of a boost.
I would consider feeding them a mash. A feed at 20% protein and 1% calcium would be ideal. Make the mash by adding warm water until the feed is wet but not so wet that you can pour it.
Tuna and even chopped meat is good.
Plenty of straw or wood shaving on the floor in case as mentioned above they can't make the perches. Temporary nest type boxes with straw in, one big one , might be an idea.
Lots of good advice in the posts above.
Good luck.