EDIT: Also, I'm guessing you already save to a Save Slot by doing the system Save action, so, It is just nonsense, again, I can be wrong
EDIT2: you can load, not from JSON, but just load, and enter the name of the slot you selected before, don't use webstorage actions At all.
Yep, that's the way I do it. No need for webstorage imo.
System - Save game to slot "Slot1"
System - Load game from slot "Slot1"
Deleting games tho? Sorry, no clue. But saving to the same slot overwrites the old, don't know if that's any help.