So sorry to hear this sad update. You did all you could and had the best of intentions, so have peace of mind knowing that. I too get frustrated when a chicken of mine dies and I tried everything, but to no avail. I presently have a hen on antibiotics prescribed from a vet and I don't think she's going to make it despite my best efforts. So things just happen.
Hang in there!!! You're not a bad chicken keeper!!!!!!!
