Answered by Borut | Jun. 14, 2023 00:01
No, unfortunately, you cannot do that because your itinerary is not a transit, but a return journey. Also, to be eligible for any kind of visa free transit / entry into Guangdong province you have to fly into it. Overland crossings are not eligible.
However, you are advised to rely on Shenzhen 5 days visa on arrival. Singaporean citizens are eligible for this visa and the price is 168 CNY. You can obtain it on several border crossings like Luohu and Huanggang, but I do not recommend them because of congestions and long waiting time. I recommend you to take CKS - Chu Kong Passenger Transport ferry, departing from Hong Kong at 09:00 for Shekou in Shenzhen where you can apply and obtain a Shenzhen visa on arrival. You should just have in mind the opening hours of Shekou visa office: 9:00 - 13:00 & 14:30 - 16:30.
To return back to Hong Kong, you are free to use any border crossing and any mean of transport. People usually take a high speed train from Futian station directly to West Kowloon station in Hong Kong.