Just a note regarding Aberforth's journal entry - he's been away doing some of that unofficial hunting down of bad men. Only Albus would have any idea of what he might of been doing though even he wouldn't have any absolute answers. It's just that he knows his brother and knows what he's likely to have been doing.
Before he left, Aberforth would have told the Order that he was visiting an old friend so that's about all they'd know.