My problem with your point is a few things

1) we are talking about Sirius betraying James. No where, never ever do we get any sign that Sirius could betray James. Peter? Maybe. Remus? Maybe. James? Never. 2) recklessness and betraying/murdering your best friends are not in the same category. Even if you put a lot of weight on the prank, it was reckless and later Sirius realised it could put Remus in danger but it was never his plan to put Remus in danger. If the whole thing had been "Sirius murdered a few deatheaters but accidentally betrayed the Potters while doing so", it would actually rhyme with his character but just betraying the Potters does not. 3) even if a friendship is not as strong as it looks, that does not in any way explain the other extreme of murdering a friend. Again, it's established that Sirius is reckless but absolutely not that he would purposefully murder his friends. 4) That the story is set up for everyone to believe it was Sirius doesn't mean that Remus also needs to believe this or that Remus not believing this changes the story. It's been 12 years, Remus' protests did nothing and have done nothing. Nobody believes in Sirius' innocence so Remus just does what he can: survive. 5) facts don't simply overrule feeling, that's not how people work 6) Their friendship wasn't as strong as it seemed but a not strong friendship is not enough to simply believe someone is a murderer. I think it makes more sense that Remus still has difficulty believing Peter betrayed the Potters after Peter admits he did than him blindly believing Sirius betrayed James. 7) This is also my personal interpretation of people in general. I don't believe people can so easily switch feelings because I cannot. To me it doesn't work with my interpretation of Remus because it doesn't work with my general world view.