I hate to admit this but many years ago when I split with my ex husband, I went back several times to spend the weekend with him. We couldn't get along living together but we were still physically attracted to each other. My folks got pretty upset with me over that, too.
Just a thought, but maybe she and her husband were still 'friendly.' Maybe she was there to spend some time with her stepchildren. That's not so uncommon. If he followed her and killed her, why would he tell cops she was intending to come back, making him look suspicious? Why wouldn't he just say she left after picking up some things and he wasn't expecting her back that night? How old were these stepchildren, would they have been home at the time? Maybe they can confirm whether he ever left that night or not.
As for possible errands at 9 p.m., it's not like it was midnight. Many people have run into town for something at that hour. I've sent my hubby to the store for milk or bread at that time. Or he has gone to the store at 9 or 10 to get cigarettes. I've run into town to mail a letter or a bill so that it would go off by 7 a.m. We live in a rural area and our mail carrier doesn't get around to our house until 4 p.m. or later. I can think of a dozen errands that someone could be doing at 9 at night.