Sirius is an incredibly damaged character and was before Azkaban as well. He's remarkable in that he came out of there half as sane as he is, but the truth is that he wouldn't be sane and - therefore - is an unreliable narrator when it comes to...everything. It's also not unrealistic to think that he was part of the reason that Regulus ended up as a Death Eater. James Potter gave him an escape route, but he didn't - for whatever reason (House or family prejudice, teenage self-centredness, etc.) provide one for his brother.

I don't usually read time travel fics to the Marauder era exactly because of the reasons you listed. Every time I try I want to slap fandom silly so it's not worth it XD All the things I'd want out of it are uniformly not there (plot, characterisation, realistic relationships...). Not to mention there's all these references to modern technology everywhere that make me want to scream.

Maaaaaaaaybe. They'd be WIPs, and I unfortunately have the attention span of a mayfly, but I'm not saying no.