What exactly are you feeding your birds? what recipe are you using?
It's normal for hens to slow down in summer.
Herbs do not cause or encourage laying, that far beyond their useful capacity.
Cut out all this stuff, it's not helpful for them
Let them in with the others, you want the chicks to integrate while momma is still take care of them and will protect them. She'll only be taking care of them for maybe two more weeks
Have you tried corid for the chicks?
Mareks doesn't affect that young of birds and I'm pretty sure it takes longer than 10 days for mareks symptoms to appear in older ones.
What's your older birds diet?
How hot is the brooder?