I think it hasn't occurred to the Grangers that Hermione might go to another school in another country yet.
I don't think Harry is trusting Bumbles. I think Harry is acting very Slytherin actually. while Alastor and Dedalus are Albus' good friends they are also both on the side of the law. Neither are going to allow Bumbles to get away with much. Remus was the wild card but Harry played on his humanity and his brotherhood. Finally he just plain showed them the manipulations and other shenanigans the old buzzard was up to. Aberforth wasn't shocked by these notice. So now he has help keeping his brother in line.