Hmm, if she had been eating a food with chicken and then happened to be sick, I wouldn't jump to the conclusion that chicken is the issue. I am not saying that you should rush to feed chicken, just that by excluding it --possibly for no reason-- you are excluding a lot of quite good foods. Lamb is expensive, foods with lamb are going to be both more expensive and have lower amounts of meat in them (and/or the meat will contain more bone which means more mineral content). Same with most other novel proteins. I'd suggest comparing the ash content of varying formulas. High ash content may be a problem for urinary and kidney function. Could be that the batch of food was bad or had molded or something else. I thought for a long time that my male dog couldn't eat pork because he had reacted to a very small amount of ham. Turns out he is fine with pork (and pork blood meal) in dog food, it was as best I can figure the salt content in the ham.
Dogs can digest grains to varying degrees, they are true omnivores. I originally thought that improvement in coat, stool etc was due to going grain-free. But, as least in the several dozen dogs that I have observed directly, the improvements are mainly based on protein content and fat content. 30/20 is a sweet spot for my dogs but each dog is an individual. Some of the foods my dogs do best on have grains-- Victor Nutro Pro, Sportmix Wholesomes chicken, Pro Pac, Inukshuk for example. Wheat /= grains. Grains can be barley, rice, brown rice, sorghum, oats, and more.
Even if you are committed to staying with Merrick overall, IMO it is a good idea to at least try 2-3 other brands. Reason is, if a brand ever has a shut-down or recall, it can be invaluable to know a backup food that your dog does well on. Even the best manufacturers can suffer something like a natural disaster or a kitchen fire and shut production for weeks or months.
Basically, I'm not trying to steer you away from Merrick nor towards any particular brand really. Just giving food for thought (er, no pun intended) that cost does not always equal quality and that there is more to food than star ratings.
Out of curiosity, why do you say that you still like Backcountry more than the formulas I mentioned? Reason being, if I know why you like Backcountry over those, it might help me suggest another food option that suits.