Looking at your very nice set up and beautiful coop. I do think your run would be better with some clutter in it. Clutter breaks up the line of sight, offers shade, and wind protection. And allows chickens to get out of sight and away from each other.
If you look at your run, you are wasting the vertical space, it is basically just a two dimensional space in which all the chickens can see all of the chickens 100% of the time. Adding some roosts out there, a wind break, and some shady spots would be beneficial to your birds. Pallets up on blocks that chickens can get under or on top of,
old ladders, mini walls out of pieces of plywood or even cardboard can really help.
But I don't think that has anything to do with the death of the bird or the feather plucking.
Mrs K