I purchased some ex batts last year-they were in horrendous condition,they all feathered up nicely and ended up with shiny plumage but several went the same way I'm afraid,I have lost four go over 5 weeks now,it seems they were probably nearing the end anyhow,two just dropped dead with no apparent conditions and one was gasping for air and her face had turned purple! she had laid in the morning and was ok.done an autopsy and did not find anything in her windpipe,very odd.
You have definitely done the best thing for them,it is horrid to see anything suffer!