Visa on arrival

Singaporean citizens are definitely eligible for Shenzhen 5 days visa on arrival and your dates are fine because those 5 days allowed for a stay start counting from 00:00 the day following the entry date. In other words, regardless of the fact that you land on 9 Aug, your 5 days start from 10 Aug at 00:00 and you can legally remain in Shenzhen until Aug 14 at 24:00. You will exit in reality some 23 hours and 59 minutes before the deadline.

The main problem here is not about the dates, but about the fact that Shenzhen airport authorities are reluctant to issue 5 days Shenzhen visa on arrival. We had an example of a Singaporean who landed at Shenzhen airport, but was refused for Shenzhen VOA, so he ended up paying an additional ticket for a third country because he used 144 hour visa free transit instead.

I would suggest you to do exactly that. Use 144 hour visa free transit by landing in Shenzhen and existing to Hong Kong by direct ferry. Please, just have in mind that you will have to book a ferry ticket before your arrival in Shenzhen, to print it out and to have it with you. You will really have to depart by exactly that ferry you have bought your ticket for.