Okay, if I had to pick one of the nasties, I would say it's an Infectious Bronchitis although you are not describing all of the symptoms usually associated with this, the way the chick is acting is consistant with this. I would say a course of the Duramycin-10 as well for your chick. Use it for the 7 days at least. The bloody droppings make me think there is a cecal coccidiosis going on as well. I would use Corid for that. If this isn't cleared up in a week you may have to switch to Sulmet.
Now, the above being said. Let me rant here for just a minute. Purchasing birds from someone who may be passionate about their birds isn't always a good thing as you are finding out. Pasty butt or diarrhea at 2 weeks of age was your first clue that all was not well with these birds. Stunted growth was another. She probably has something in her flock that #1 she isn't aware of or #2 is aware of and doesn't care. I too am passionate about my birds as well, would you be interested in getting some of mine? I have Marek's in my flock and now either an IB or MG as well. See where I'm going here?
I am a little puzzled about the 2 that aren't showing symptoms. At a guess, and that's all it would be as I'm not a Vet, I would say she has some resistance built up in some of her flock. That is if I am correct in thinking it may be IB your bird has.
Now, the above being said. Let me rant here for just a minute. Purchasing birds from someone who may be passionate about their birds isn't always a good thing as you are finding out. Pasty butt or diarrhea at 2 weeks of age was your first clue that all was not well with these birds. Stunted growth was another. She probably has something in her flock that #1 she isn't aware of or #2 is aware of and doesn't care. I too am passionate about my birds as well, would you be interested in getting some of mine? I have Marek's in my flock and now either an IB or MG as well. See where I'm going here?
I am a little puzzled about the 2 that aren't showing symptoms. At a guess, and that's all it would be as I'm not a Vet, I would say she has some resistance built up in some of her flock. That is if I am correct in thinking it may be IB your bird has.