Lacey is found by her grandmother Joan passed out from DKA, one of the many consequences of untreated type 1 diabetes. Most likely though, other symptoms (nausea, vomiting, excessive thirst, rapid breathing, rapid heartbeat) would be in evidence first. Loss of consciousness would only occur in severe DKA; her BG level of 450 is serious but wouldn't be at the level to cause LOC.
In a death investigation, the next of kin always makes an ID of the body, which would have taken place at the hospital since the parents of both girls were there. The girl shown as dead had some facial injury and blood on her face but this would not have prevented her family from recognizing her.