Even if people who fly are less superstitious, that doesn’t mean they’re not superstitious—maybe an even higher percentage of the general public would have skipped their flights. This doesn’t provide an alternative explanation, or make the author’s explanation any less likely.

This just doesn’t give us enough context to know how it relates to the 17 percent of people who missed their flights on Friday the thirteenth. Maybe only a very tiny percentage of people fly, and they’re unusually likely to be superstitious—we don’t know.

This weakens by providing an alternative explanation for why more people missed their flights on Friday the thirteenth. If there happened to be severe weather those days, it makes sense that people would miss their flights even without superstition being involved.

This does not weaken, because the domain of the argument is specific to flights. It would be perfectly reasonable for people to fear flying on an unlucky day but not fear driving, because driving is a much more routine activity.

This does not weaken, because there are other factors that explain why airline workers might show up to work on an “unlucky” day. So this doesn’t really contradict the observed data about passengers, and still doesn’t explain the phenomenon.