I really feel you on this, having a 7 month old with persistent cough and congestion is so exhausting and worrying, especially after a KKH visit. What helped my little one was elevating one end of the cot mattress slightly so the head is a bit higher, which makes it easier for them to breathe at night and helps the mucus drain a little. For the humidifier, I would be careful with essential oils for such a young baby as some oils can actually irritate their airways, so it might be worth checking with your PD first before using any. Since your baby is still so young and has already had that drop in oxygen levels, I think it would be good to loop in your doctor again just to be safe, especially since the coughing at night is still going on after a few weeks.