So on with the next part: last time we found out that a half-giant with little to no magical expertise could break through the Fidelius Charm of a heavily protected house to retrieve a one year-old child, which came to be regarded as the saviour of the world later on by a select group of people known as wizards.
Based on everything that I have learned in the Harry Potter series about the Fidelius Charm, the retrieval of Harry should not have been possible. In theory, you can destroy everything on the inside of the Fidelius Charms range. But that will not alter anything that happens outside that perimeter and will not change the nature of the secret that is kept. Say that, for example, I decided to Fidelius my own house and to confide that secret to only one person, then, apart from me, only one other person would know that the house was still there. The rest of the world would remain at large of where the house is or whether it even exists. Let’s say that I then decided to ignite several kilograms of TNT inside the house. Please note: the surrounding would have adapted to non-initiated persons to be completely normal, except perhaps for a missing house number. If my Fideliused house would be gone thereafter, nobody would know it isn’t there anymore except me and that person, and none of the surroundings would be affected by that explosion of TNT. For any other person, the street should therefore still stand as it always did. I would be able to go and see what happened inside the house, and so would the secret keeper, but the Fidelius would not expand to others.
As long as I don’t get to know who was confided into the household of James and Lily Potter, I will consider the retrieval of Harry by Hagrid a massive plot hole, bigger than the house left in the street after Voldemort blew it to smithereens in his botched killing attempt. Which nobody seems to have spotted either, so far.
As soon as Sirius Black knows what has happened, he goes after secret keeper Peter Pettigrew. Peter fakes his own death and blows up the street around him as a ruse to get away from Sirius. Seconds later the aurors (which are whatever passes for a wizarding police force and peacekeeping army) show up. Naturally Sirius is apprehended and taken into custody. However, he never gets a trial. In the third book it is recounted how this all ended.
In 1993, it seems to be common knowledge amongst the higher echelons of society and the Hogwart teaching staff that the Fidelius was involved in the protection of the Potters. Before that, at least Dumbledore, Sirius, Peter, James and Lily would have known that the Fidelius was involved. Any attempts by the ministry to locate the Potters after the vanquishing of Voldemort would have shown their searches to be fruitless, as they were in Godric’s Hollow, Potter Manor, and stayed there.
One can argue that Snape was amongst the circle of initiated, but he will be discounted for his hate of Sirius for now, in spite of what the 8th movie shows.
Only the death of the secret keeper transfers the responsibility thereof to the circle of those knowing the location. The secret keeper never died though and there is no known procedure to cancel it. So, for all clarity, the Fidelius was still in effect. Blacks innocence on the betrayal of the Potters could clearly be proven after asking him to divulge the location of Potter Manor. (Assuming of course that the Potters are of high enough a standing to have a manor.) The sodding idiots in Azkaban or the ministry never did. They would have little reason, but Dumbledore or any other person knowing where Potter Manor is could tell you that Peter Pettigrew logically betrayed the Potters because he was the only person capable of doing so. If you would have asked Sirius to tell where Potter manor is, he might not have been able to tell you and this would have given the indication that the secret keeper is still alive.
As all wizards seem to know each other, it is possible to discern the social circle of James and Lily to see who could visit the manor and who could not. If Sirius did not betray the Potters, you might want to reconsider the possibility that Sirius didn’t blow up the street, or at least change the circumstances to allow for excessive violence against the betrayer. It is very easy to find out that Sirius is innocent, as I may have hinted at. Let me spell it out for you: the connection between Sirius and James falls just short of them marrying one another. Their friendship is really strong. It is known that Sirius loathed the dabbling into the Dark Arts of his family. He did enough to distance himself from his family, and his family did enough to have never considered Sirius a member of the family ever since. Joining Voldemort’s cause therefore seems to be unlikely, lest he had a change of mind. Voldemort was taking over, so you could doubt his allegiance to the non-dark side.
However, his character is only a minor aspect. As soon as Voldemort had vanquished, everyone in the wizarding world knew of it in no time. Perhaps even within hours, as the very same night showed falling stars on account of mr. Diggle celebrating the downfall of the Dark Lord. Information travels very fast apparently. Now everyone also seemed to know the Potters had something to do with it. As is normal for humans, they immediately swarmed ground zero, the place where it all happened. As it is under a Fidelius Charm, everyone outside of their circle would have noticed that the manor couldn’t be located or even accessed. If they could not have been found, it would have become clear that the Potters did not wish to be found and had taken several measures to prevent localization of their manor. One of these measures was the Fidelius Charm, known to several people, including the supreme mugwump himself. As nobody could enter Potter Manor or even find it, Dumbledore could have happily concluded that the secret keeper was still alive. Those capable of entering Potter manor knew that Pettigrew was the Secret Keeper.
So either the Dark Lord broke through the Fidelius by unknown means, or he got the information from Pettigrew. Dumbledore therefore should have known that Peter is responsible for their betrayal, or that some measures regarding the Fidelius Charm had to be taken to ensure better protection against future dark lords.
As soon as Sirius was apprehended and thrown into Azkaban, Dumbledore would have known about this because of his position in the Wizengamot and general position as a beacon against darkness. He was a respected man by that time. Dumbledore should therefore have been one of the first people to be able to declare the innocence of Sirius Black, Remus would probably have been a close second one, and any other person close enough to James and Lily to be on visiting terms would have known as well, as they still would not have been able to disclose the location of Potter Manor to anyone else. Sirius seems to have been big news back then.
You can all see where this is going. Feel free to further speculate about wizarding stupidity and their lack of knowledge about right and wrong and ponder what would have happened if your childhood dream of being a wizard in the world of Harry Potter is still an as merry dream as you thought it was…