I don't doubt that Slughorn won't come down to the Three Broomsticks (he can remember when it was just One Broomstick, after all), although he'll probably be buying more than he actually drinks. Not that he's buying for students at all!
MacNair wants to know how long ago it was she left Hogwarts- he's been out for 6 years now, and if she still has siblings there I can't imagine that it was all that long ago. If they knew each other in school it might be interesting for him to drop in on her now he's back at Hogsmede again. On Official Ministry Business to!