Well none of this is accurate obviously as we can only speculate but logically it would make no sense for Peter to have gotten the dark mark while spying for voldemort. It would’ve been a dead give away and very easily noticed. So I would go with that being the reason no one saw it. Because he didn’t have one.
and that is more than likely the reasoning they would tell themselves about sirius not having one. Because he was a spy and that would’ve been too obvious.
As for going to prison tho, the entire event unhinged him. Made him a little mental, part of him never denied that he killed Lily and James because he believed he had. It was his fault, in that frame of mind, that he’d gotten them to switch secret keepers (something that he thought would protect them further) which is what ultimately got them killed.
He didn’t fight it, he went to azkaban immediately (laughing hysterically mind you) without trial and without hope because the only people who knew anything about them switching secret keepers were dead or pretending to be. All evidence pointed at him.
But then the Black temper played a part once he was there, anger at Peter for what he’d done cut through the rest eventually and gave him the strength to keep his mind all those years, the hope to one day avenge his best friends death and the drive to break out and try.
AT LEAST THATS HOW I LIKE TO THINK IT WENT DOWN. Maybe we’ll get more insight to it when they finish PoA on Pottermore =D
He remembered all the good times he had with Ron and saw how truly terrified and heartbroken his former master was and it brought back a flood of other memories that were of the mauraders. I think that his death was a suicide in a way because he couldn’t deal with the guilt of betraying those closest to him.
That’s pushing a little too much), he wouldn’t have befriended James, and thus he wouldn’t have betrayed him and caused his and his wife’s death. I blame the Sorting Hat for making a mistake and ruining Harry’s life. (Ok not really, but still. The Hat has it’s share of blame.)