Here is a picture of her taken today:

And here is one taken the first day I got her:

She had some similar raggedness when I got her, as you can see, but I assumed that was from a bit of plucking, as she was clearly the "least favorite" of the group. However, since I split my four birds into two separate flights, I've noticed that she's not being plucked at all (even despite the fact that she's been in a cage with The Devil--see this thread). Instead, the raggedness continues to grow and get worse looking despite The Devil working hard to preen poor Blotchy.
So I can only assume Blotchy has some kind of mites (either scaly face or feather mites maybe?) or some sort of deficiency? Since this is the very first time I've owned finches and all of the birds came from the same cage at the store, I didn't know that I would need to treat them with anything in particular.
Now I think I need something more than regular mite spray (which I do have and have used--but maybe not enough? When the directions are pretty much "Spray the cage with birds in it," it's really hard to judge!!). I have read many times on this forum that Scatt/s76 are pretty much a must for mites and parasites.
However, reading through all the material on them is making my head spin. I have four birds and they PANIC if I so much as attempt to catch them, so I'm worried that Scatt will be difficult to administer, and is it effective on external parasites, in case it's feather mites and not scaly face? I have conflicting answers on that. On the other hand, is it sensible to get s76 and put it in the water for however many days are required if all I have is four birds?
Advice very much needed!