I wonder if LE has enlisted the help of a physchologist to interview everyone. If not, they should seriously consider it. I'm of the opinion that if your best sources, meaning friends, family, and neighbors, are in denial or worse, LE is never going to get anywhere. I don't mean this to come off...
I think BD protects SA because she "has" to believe in his innocense. Otherwise, she would have to face the possibility that she brought her daughter's attacker into their lives. Not blaming BD, just theorizing on why she may feel compelled to protect him, even now.