For your particular situation it isn't so much about adult vs. chick but the fact that you are dealing with a hen with previous trauma who is now in a new setting and, for the sounds of it, alone. Chickens are, by nature, a flock animal - there is security in the flock and insecurity in being alone. Being a lone bird is going to have her in a heightened state of awareness/reaction. The previous traumatic experiences will also have colored her perception of things going on around her.
All that being said, I dont' think this is a hopeless situation at all. Most of my childhood flock came to us as adult birds, and all were my pets. With time and penitence you can overcome this and have the relationship you want with this bird. I would suggest not approaching the bird, but focusing more on just being a presence in her surroundings that becomes familiar. Take a chair and book and just sit with her. Ignore her and do your thing. Become a part of the scenery. You can start scattering choice treats in the area around where you'll be sitting, working the treats closer to you as time goes by. Let her approach you. When she does approach you, don't immediately try to touch her, let her peck at you, look at you, walk around you or even ON you, and just be still. When she is comfortable in your presence you can start adding light touch to hand feeding, so that the association of touch is a positive one that is linked to eating special treats while being touched. All talking should be in a low, calm voice - all movement should be slow and non-startling.
