This is my save game 101
Notice how, every time you want to save something, you have to read the save game first.
This is so that you write it back out with the stuff it already had in it 
Regarding your other question: yes, a centralized approach to saving is a good idea. Do the save game stuff in a central place like the game instance, or game mode, then you don’t have to worry about organizing it anywhere else.
The game mode ( or instance ), can have custom events ready, one to give you the SG variable, one to save it etc. Also, if you do that with interfaces, you don’t even have to cast