He may be dumb, but he's not that stupid. He doesn't strike me as brazen enough to bring the mistress to his house when the kids were there (or to kill the kids then have hot sex at the murder scene). I would say it is VERY likely that SW had friend spies checking to see what he was doing while she was in NC. And I wouldn't doubt it if he brought the mistress to their house when she was gone in NC for weeks. It's very possible, that since she suspected the affair, when she was in AZ, it was confirmation of it, maybe there was a confrontation.