Also, I want to repeat that the depiction of retrograde amnesia here isn't fake at all, just rare. Most cases even of amnesia that covers a period of 3 or more years they gradually recover some memories, and often most. It is the case, though, that the longer-period retrograde amnesia has more to recover and therefore is less likely to recover it all. So Arisa won't be restored to her old self, the long time it takes to recover about a third or so of her memory, which is all she'd be likely to get back, will be interspersed with new memories. They've even had overnight recoveries of most of the memories like in Watashi no Taisetsu na Tomodachi, but that's not the norm.