1. No, you don't need visa. Valid Singapore passport holders are exempted from visa to visit Macau and cities in mainland China.
2. Almost all shops in Macau(https://www.travelchinaguide.com/cityguides/macau/) and large shopping centers in Zhuhai, Guangzhou(https://www.travelchinaguide.com/cityguides/guangzhou.htm) and Shenzhen(https://www.travelchinaguide.com/cityguides/guangdong/shenzhen/) will be opened all the same. But, these local wholesale markets will be close during Chinese New Year(https://www.travelchinaguide.com/essential/holidays/spring-festival.htm).
3. There is only a few of buses driving to Guangzhou from downtown Macau, also have to pass by Zhuhai. Comparatively, there are much more buses departing from Zhuhai to Guangzhou.
4. Shenzhen could be reached by ferry from Macau in one hour. At Macau Ferry Terminal, there are ten ferries to Shenzhen Shekou Port(https://www.travelchinaguide.com/cityguides/guangdong/shenzhen/shekou-port-ferry.htm), the fare of which is CNY170/adult. You'd better go to check the ferry schedule one day in advance, because the schedule may be adjusted during holiday.
5. Generally, the weather of Guangzhou and Shenzhen is almost the same. It will be warm days, comfortable for traveling outside.