If not dinner, it could have been another outing. It's not unusual for business people to go out for dinner at 9:00, especially if that's when they get home from work, and the household help is away, and they had a meeting over the dinner hour. I don't see where they might have had dinner in sooner than that.
Or they might have been heading out for another purpose, or not. The main point is that we can't be sure that HS was murdered earlier based only on the fact that she was wearing a coat. BS might have called her ten minutes earlier to say he was nearly home, and asked her to grab her coat.
That is an interesting point. If they were hog tied together or even if their wrists were bound, it is hard to imagine why the killers would do it if Honey was dead or near dead. Three hours of who knows what happening to Honey does not seem like she would be in good shape.
I can see that perhaps someone hid in Barrys car.