I agree with others that what you were told by the feed rep did not suddenly kill your keets and chicks. But something in the feed could still be toxic. Others have mentioned mold, & aflatoxins produced by mold arent always visible. Your words "the feed was "personally mixed in the store and ingredients written on bag", raised a huge red flag in my mind. Toxins other than mold could also have somehow been added/mixed in.You are correct it is not a layer feed, the layer feed was in completely different bags, it was personally mixed via the feed store and was in all white feed sacks, sewed shut with ingredients written on the bag.
Others have mentioned sending off a dead chick for necropsy to determine cause of death. Another avenue is to contact your local county extension agent, explain what happened, and request a feed analysis. Seems to me that the feed rep is covering their a$$.