Using pesticides as a preventative can possibly cause resistance.
best to only treat when pests are present and positively ID'd.
Neither of those will do much...and garlic can cause anemia.
Yep. Cocks here have busted up tail feathers because I put the roosts too close(12") to the HC walls. Nothing will 'fix' them...and it's really not an issue unless you are showing the birds.
As already stated, you can't fix broken feathers.
But you should figure out why they have broken/split feathers.
Some pics of said feathers would help here.
Additional info needed:
Number, ages, and genders of birds you have.
Dimensions and pics of coop and run.
What all and how exactly are you...