Vomit is obviously unpleasant. If someone collapses from an asthma attack, it is because he or she is not getting enough oxygen. This is because all the lung's small airways have narrowed and are not allowing enough air to reach the air sacs. Mouth to mouth respiration may help a little. Usually CPR in infants and children is performed for respiratory arrest such as severe asthma.
Ventricular fibrillation is rare in children but very common in older adults.
In regards to administering the Heimlich Maneuver to a victim while they are lying down. Since the airway is blocked you shouldn't spend much time positioning the head. Yes, if you give assistance, including CPR, for a medical emergency Good Samaritan laws cover you. When the heart stops beating in cardiac arrest the breathing center in the brain is still alive for a couple of minutes and will cause the victim to take a few abnormal breaths. These abnormal breaths associated in dying are called agonal respirations.