How would they know if his occlumency barriers were down?
It is pretty clearly established that if you are sufficiently skilled, you can make a Legilimens who tries to read your mind see whatever you want them to.
So if harry said he let his occlumency barriers down and he was telling the truth, they would see his barriers down and see whatever he remembered seeing. But if he was not telling the truth it would still look like his barriers were down and they were seeing his memories.
Harry is already good enough that he does not think veritaserum would be able to have any effect on him, so it seems unlikely that anyone could have been sure if he was telling the truth even if they saw in his memories that he was innocent.
How would they know if his occlumency barriers were down? It is pretty clearly established that if you are sufficiently skilled, you can make a Legilimens who tries to read your mind see whatever you want them to. So if harry said he let his occlumency barriers down and he was telling the truth, they would see his barriers down and see whatever he remembered seeing. But if he was not telling the truth it would still look like his barriers were down and they were seeing his memories. Harry is already good enough that he does not think veritaserum would be able to have any effect on him, so it seems unlikely that anyone could have been sure if he was telling the truth even if they saw in his memories that he was innocent.
Edit: looks like someone already said this