Some brahmas feather funny, they are a slow feathering and maturing bird. I think you have a pullet. You'll know if the comb grows and reddens in the next couple weeks. That comb is still pretty yellow. I see pink at the tip only, but I'm looking on my iPod touch, so maybe I'm missing something. I would hang on to this one for a while.
So sorry to hear of you bad experience. I buy mine from a local store, they ship the chicks in and some are sick with respiratory issues. I always give poly vi sol vitamins with probios and I have not lost a chick yet. Last year my SS got really sick from knocking the waterer over and almost died, she spread her sickness to the other birds, but they all lived. Normally people would cull, but I'm not a breeder.
I do use DE and Sevin with the big girls, but only as a preventative. I haven't seen mites yet. I do let them free range and they do find dead mice and birds, so I try to keep up on dusting them.