Great! Just divide each into three and that'll be okay.
No, I wouldn't sprinkle it in all of their food as you won't know how much each one is getting and she's the one that needs the extra, they don't.
To put it in food, I'd take a third of it and put it in a small amount of mash (crumbles...
It can take a day or up to a week for the parasites/worms to die and be completely excreted. It's also recommended, if you used Albendazole, that it be repeated in 10 days.
As long as she's still acting well, eats, and drinks, etc., I'd stick with this through completion.
You'd want to follow...
The two top reasons for feather picking are overcrowding and lack of protein, neither of which is the case with yours.
Are you sure they aren't molting? You mention some of the feather picking you have seen, but much you don't. That might explain why one isn't losing any feathers but others...
Oh honey, I'm not sure where you read to give a selenium supplement other than scrambled eggs, but it'll be okay. Treatment is usually only for a couple of weeks, so I'd go ahead and put it in their water, as that's easiest, provided it doesn't taste nasty. If it does, you can add a little bit...
I'm always thinking my silkies need extra everything, but I'm also careful of overdosing. If you use this, that will undoubtedly give them the max or close of the selenium they should have in a day.
I wouldn't feed them anything with egg yolk in it then if you're going to use this. Also...
It looks nice. Not sure why more insulation was needed as it appears to be an indoor coop already.
Some information about how it was built, materials used, etc. would be helpful for someone wanting to build it.
Excellent job explaining how to build this brooder.
The only thing I would change but not take stars off for it is I'd use 1/2" hardware cloth. That keeps out most everything.