InvaderX
I can't help, but do indexedDB storage (used for the save-states) isn't Really Huge?, and so used instead of WebStorage because of that? (so, I'm not sure to store it entierly in the webstorage is a good thing to do).
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.