Dziękuję, teraz już rozumiem w czym rzecz. Przepraszam, ten brak inwersji po seldom to mój błąd, źle zapisałem.
You might learn quite a lot about negative inversion if you tried to construe your first example with no inversion despite the fronted "seldom." How would you read out loud the sentence if there were (invisible) commas present after but
There were a lot of tourists even those days, but, seldom, somebody happened to wander off the main track.
To ease your mind into thinking in that direction, take a look at the two strongly negatively fronted sentences below:
With no husband will Peg be happy. (with negative inversion)
With no husband, Peg will be happy. (no inversion)