So not being at fault for his mental illness, and not being legally liable for his actions, and being fully cured, and (in our take) having no memory of anything that happened after the accident, Starfleet returning one of their most important captains to service is not unlikely in an enlightened universe that Roddenberry created.
Sorry, but you are also saying that the very nature of Human beings have changed as well. A "truly sane" individual that knows they took such actions would be traumatized by it - and moreso by the amnesia of such events. Because their guilt would make them imagine worse than people know about.
I've treated people who have amnesia, and even when there is no horrible event in their past they are traumatized and are sure they are guilty of something no one else knows about. If Garth is a decent Human being he would be seriously screwed up after being cured - forever. If that is not reflected in his story after the TOS episode, it's just silly.