“I am not convinced that the hoses will flex enough. Why does everyone winterize water systems then?”
Because your water system Is comprised of pumps, ridged lines, hot water heater, faucets, valves, and flexible hoses. Skip the winterization process and you will understand completely
“Regarding the shop vac, why would this be able to pull water out when the freshwater pump itself can't?”
You stick the shop vac suction hose into the water tank and vacuum out the remaining water. Which is Probably not necessary.