When arguing about what to do, one of the characters responds to a suggestion that they all just stay in the house as unrealistic and unhelpful. When Em finally finds a "happy" reality, everyone has actually just stayed inside. Turns out that was a pretty good solution.
The movie only shows us how the people in one house handled the comet's passing but there is no reason to believe they were the only ones affected. There are presumably many other houses on that street that were directly beneath it and probably several adjacent streets that are equally as populated, since it doesn't appear to be a rural area. Once you multiply that population across the different realities it suddenly becomes very likely that countless thousands of people.
Even though nothing bad happens in the happy house that Emily finds (apart from her attempted murder), all of the bad things that were uncovered in the other houses are still lurking under the surface. Emily's dancing career has stalled out. Kevin and Laurie still hold candles for each other even though they're dating other people. Beth still cheated on her husband and never came clean. Mike still has a drinking problem that ruined his career and is affecting his marriage.
Em mentions a Finnish woman falsely remembering having killed her husband. In the end, this is explained by the comet's multiverse effects, meaning that she killed the husband of another version of her, and that version of her likely took the fall for it.
