How to get from HK island (Sheng Wan Area) to Shenzhen Airport for a 22:15 flight?

You can take a taxi to Sheung Wan Hong Kong–Macau Ferry Terminal first. Then you can take the shuttle bus there to the airport directly which departs at 18:00, 18:15, 18:30, 18:45, 19:00, 19:30 and 20:00. The ticket fare is HK$130/adult. The entire trip will take around 2 hours. Considering your situation, the bus leaving at 18:00 or 18:15 is quite suitable for you.