Multiple non-(lab total) lab activities?

Because one day is not a season, and there is, in many cases, nothing for the second one to penalise.

Yes. They might be simple, but they take time. The same applies to all simple tasks, of course. The magus can do it in a season at a -20 penalty, so it goes from all to nothing at 20 to 21 days.

Well, the third with the correction is not the version on the poll. The third option on the poll is definitely wrong.

Season - 20 or season - 10 are both reasonable interpretations. The essential point is that it should be impossible to do any other seasonal activities at the same time, or to do more than one of these activities in a single season.