the natures bounty only has 20 mg niaciamide per 1 ml but the durvet liquid b complex has 100 mg niacinamide per 1 ml ...
You would have to give 5X more if you use natures bounty .
you can get the durvet on amazon or at tractor supply or probably any farm and feed type store heck even ebay...
since they ate treats did you provide grit? they need grit ( chick size grit at this age) in order to be able to digest anything that is not their regular feed.
use the nutritional yeast, 1 Tbsp per cup of feed . i would also get liquid B Complex and mix in 1 ml of it with a little tiny bit of feed, like make a little soupy mash.