60 degrees isn't cold to them. During winter, you need to put straw or something down to keep them from the draft.
At this point, it doesn't sound like the cold is the problem. Is there a chance you could take one to a vet for general disease testing?
I know that I don't worry about bees in the garden. I leave them alone, they leave me alone. I'm risk averse, though, and don't see a good reason to take extra chances, especially with the price of an epipen!
I really think you need to get something to insulate the floor. All of her heat is going out of her feet since she can't keep them warm properly with cold air coming up from below.
The food sounds good, so I would try adding another hiding place in there if you can.
It won't hurt to check for mites, though if that were an issue, I would think that the male would be showing signs as well. How old are they?
Oh, I don't see a dirt/sand bath in there. Do you provide one...
In my experience, I would say that they don't have good night vision.
I believe this because 1 - They know me, but if I come in at night, they freak out. 2 - If I'm quiet, I can sneak up on them and pick them up before they see me.
You want at least 1 square foot per coturnix, but remember to subtract the space used by feeders, waterers, sand/dirt bath, and hiding places.
You're planning on using 1/2" hardware cloth for the wire? Remember that everything likes to eat quail, so it really has to be predator proof. You...