First on the spices. Will these things hurt them? No, not as long as you don't get silly in how much they eat. If your waterer is metal I would not put vinegar in the water as that can rust out the waterer. Will these things help them. Many people believe so but I don't know if there is any real benefit to the chickens or not. I think the biggest benefit is how it makes the person doing it feel better because they are doing something special for their chickens. There is value in that. As many of us as there are that don't do this I don't consider them necessary. I do not use them other than in what they get in table and kitchen scraps.
If you deal with living animals you will also deal with dead animals. Chickens die. Sometimes you can figure out why, sometimes not. It could be some type of disease or parasite overload. Maybe a birth defect that was OK until it wasn't. Maybe an accident, it flew into something and broke its neck. Maybe the others killed it or a predator got it. A lot of times you just don't know why. You want to know why of course so you can fix a problem if you have one, but just finding a dead chicken as an isolated case does not mean you have a flock problem. Finding a second dead chicken soon after is a huge warning.
It is disconcerting that the body would be left there to decompose. To me that is just wrong. The smell can get awful. It may attract predators. I would not worry about them eating maggots or such off of the dead body, some people set up a "Black Soldier Fly Larva" feeding station to feed them maggots. The idea of the other chickens eating rotting flesh bothers me. I just don't like a dead body being left in there.
I don't know how long it has really been. It should not take that long for the body to dry to a husk unless it remains frozen. They poop a lot from the roosts, it would not take that long to cover the body with poop. So take this with two grains of salt. Quarantine is usually recommended to be a month long. If the body has been there a month they have already been through a quarantine regarding whatever killed her. I still think a quarantine is a good idea. Getting a fecal float from the vet to check for parasites is a good idea. If it has been a month I would not consider these hens any different from any other hens I was bringing in from a place they may not have been well cared for.
I agree with
@nuthatched what they need is good feed, clean water, and decent living conditions. They should prosper under those conditions.