Mortality rate?

I live in Northern Virginia and our temperatures have been really chilly for the last month.  No frosts but the nights are still too cold for tender things. 

For my first chick experience, I made my brooder out of a 45 gallon storage container with the heat lamp, and put that inside a big wire great dane dog cage (I have 2 big labs who would LOVE to love chicks!) Nice and safe and warm.

I got first 8 chicks (2 Australorps, 2 Salmon Faverolles, 2 Barred Rocks, 2 Blue Andalusians)  on April 10 from MPC and 1 Barred Rock was DOA.  One of the Blue Andalusians had a broken leg and died within 24 hours.  I ordered 4 more chicks (2 Buff Brahmas, 1 Light Brown Leghorn, 1 Salmon Faverolles Rooster)  which arrived on April 24.

The older chicks were moved out to the barn into their new coop with a heat lamp 5 days ago.  My surviving little Blue Andalusian hen got her toe caught joint in the coop floor and I found her dead within an hour of moving them. 

I moved the 4 new babies out to the coop 2 days ago, to the enclosed top section, with their own heat lamp.  Since it is enclosed although drafty,  the chicks always seem to be comfortable.  Of these 4 chicks, I've had 2 with chronic pasty butt and I remain vigilant with them. 

Last night one of the Salmon Faverolles hens slept off to herself, away from the other birds and I found her dead this morning.  Could she have just died in her sleep by not being warm enough? 

I have now gotten to the point where I am checking the chickens every 15 minutes and I am wondering how many more will die before they are mature enough to hold their own. 

Do chickens just keel over and die for no reason?

We're having a very warm day so I had to open up the top part of the coop so the little week old babies wouldn't be too warm.  I have thermometers everywhere and the only place anyone can get that would be considered too cold or drafty is in the bottom section where my 3 week olds are.  The Faverolles hen who died last night was off by herself for some reason, not with the others.  Even when it dropped to 40 degrees, they all stayed together through the night, just outside the ring of the light of their heat lamp.    I just find it curious that this chicken was so robust yesterday, has been completely normal up until last night.  She was a good 2 feet away from the rest of the flock in a sleeping position, as if she simply died in her sleep.

Everyone else seems to be doing really great.  The little babies are no longer experiencing pasty butt.