If they only hatched two weeks ago, they are not ready to go outside yet. I would wait until they're at least 6 weeks old.
Where in CA are you? There is a big difference in climate between say, San Diego area, SF Bay Area, and further north CA in the Sierras.
There is no such thing as too...
These are all the new birds that you got, right? It sounds like some kind of infection is running through them. Are they quarantined from your original birds? Hopefully it won't spread.
Since the one chick recovered, the rest may too, but making sure that they are warm and well fed will help.
Please do not put coturnix quail in with your buttons. They are likely to harm them. Chickens may even eat them after they kill them. Another thing to worry about would be disease. Chickens can carry diseases that they are resistant to but that quail are not.
Ah. I believe in that post you were looking for a source in the UK. I'm not familiar with the regulations and laws there.
You may want to get them tested to see what kind of worms they have first, though. That will help you know which dewormer to use.
About? Since this is the quail forums, I'm guessing about quail, so you may want to start here:
https://www.backyardchickens.com/articles/categories/quail.32/
At those temperatures, they should be fine outside. Make sure that they're dry and out of the wind. Button quail are not particularly cold hardy, so make sure they have heat if the temperatures drop below 40.
What is the nutrition information on that feed? Every time I've seen dove/quail feed it has been very, very low on protein, which could be causing malnutrition which could be contributing to the problem.
Are we still talking about this? Timestamp 3:40 should answer any questions of what the peach stands for...
Ending credits for Hanazakari no Kimitachi e (I love that show)
It sounds like something neurological or a head injury. Is it possible she flushed and hit her head? This is a very common injury for quail.
Another question is, what are you feeding her?
Is freezing it an option? If you put it in containers that fit in the freezer, you can freeze it for a few days, take it out for a day or two (so any eggs might hatch) then freeze it again. That will usually take care of any bugs that might be in it.