Regarding regular vs. Braggs ACV - I don't know how you would tell the difference on your chickens.
I have been using ACV for me - for 2 recent bouts with food poisoning or similar. In the first instance, I got sour burps and the runs, then remembered ACV from all these chicken posts and decided to try some Braggs for me. I recovered with no vomiting, and was only slightly off the next day. In the second instance, I was traveling, and did not have Braggs ACV with me. I started feeling sick and got the runs. At about the same time in "the progression" I took standard ACV from a regular grocery store (store brand). I made it the additional 1.5 hrs home, but was still ill along the way. Once home, I vomited once before I got some Braggs ACV into me. Then I recovered - no more vomiting - but was still pretty ill the next day - drowsy and in bed all day.
To me, the difference between the Braggs and the store brand was pretty significant. I felt like I took each at about the same stage in the illness. I thought Braggs was a miracle, whereas the store ACV didn't seem like it really helped much at all.
My 2c from personal experience. YMMV.
PS. I will use up that store ACV on my chickens just for the heck of it, as I have no need for it. But if any of my chickens get sick, they're getting the good stuff.