gonna comment instead of bothering with a new devlog - there's some autosave feature within EA's code, unimplemented if that's an autosave the way we think of it, that resets the save slot ID to 0 if you loaded in to a lot, went into manage worlds, and loaded in again.
it does give the correct ID if you've loaded in for the first time or if you've saved since you last loaded in - i'm now experimenting with not allowing my variable corresponding to the save slot ID to be set to 0.
potential issue though is whether Slot_00000000.save is a valid slot?