All of this is subjective so yes, we may portray certain aspects of the boys differently. For instance going back to the whole frick and frack thing. I view that relationship differently then others so when I write the two of them, it may be differently then someone who still pictures that relationship from before.
I think everyone (including myself) are able to suspend reality when it comes to certain stories (especially AU) because that's what AU is all about. You can make Brian a villain who lopes off people's heads for fun and then feeds them to his rabid monkeys but I think we'd also agree that while Brian wouldn't do this in real life (I hope!) unless there is some of his personality in there, it's not Brian. Maybe he does his annoying Micky Mouse impersonation before he kills them. Maybe he prays to God for their soul and then kisses his wife as soon as he's done doing the deed. Whether we all agree on how they are in real life or not, we all do have these perceptions of them. We all know Brian loves his family and is religious. He's hyper but also business minded. These are things whether we enjoy about him or not, we know as fact. So if some facet of that is thrown in, it makes it easy to ignore the rest.
You could also take who wrote it into consideration. When you go in knowing what the writer thinks about the boys and their personalities that helps as well. Like I know you hate Kevin (still don't know why. I know why you hate his voice but not him lol) so obviously he's never gong to be the same kind of Kevin I would write, but I'm sure I would still recognize some traits in your Kevin that would be Kevin across the board, if that makes sense.
Do we really know how they are in real life? No, but we obviously have a mental image of how they act and even if we agree or disagree on things, we can still believe certain elements of their personalities. If you make Brian a serial killer with absolutely no recognition of Brian then it's just a guy named Brian. I would imagine if you're writing a BSB fanfic, you'd want to at least try to include something that makes them who they are, if not then why not just write an OF?
And as you were saying before, even if we perceive them differently it can't be so completely different that he couldn't be recognized at all. I would find that very hard to believe.
Now, do I enjoy reading things like that? No lol but that's just my personal preference.
On a side note - what are you still doing up? You have a long ass drive tomorrow! Go to bed! lol