"is there any particular reason your autosave is using Save.slots instead of Save.autosave for its location?"
It's been ages since I coded that, but it probably was due to having access to code that referenced how to use JS to overwrite a manual save slot but not how to use the auto slot. Using the auto slot would make it more intuitive, but losing manual slots isn't really an issue since the game tends to lag down a lot when several slots are currently being used.