Later, in Pasuk 27, the Torah sentences a Ba'al Ov and Yid'oni to stoning. Why here does it mention only Kareis?
Rashi (in Pasuk 27): If there are witnesses and warning, the punishment is stoning; here it is discussing Meizid where there was no warning, in which case he is only Chayav Kareis. Whereas if he sinned be'Shogeg, he brings a Chatas.
Why does the Torah present Ov and Yid'oni in three different locations in Kedoshim - the initial La'av in 19:31, Kareis here and Sekilah at the end of the Parshah?
Oznayim la'Torah: It must have been very difficult for Yisrael - who were free of Mitzvos in Egypt - ro suddenly be subjected to Taryag Mitzvos together with the severe punishmentsa both in this world and in the next for those who transgressed. Therefore Hashem prsented it to them bit by bit - 'min ha'Kal el ha'Kaveid' 1 (from the light to the heavy) in a loving way, 2 in order to minimize the pressure.
See Oznayim la'Torah who elaborates at length.
Oznayim la'Torah: And it is by the same token that Hashem divided the Parshah of Arayos into two - the prohibitions in Acharei Mos and the punishments in Kedoshim - to give Yisrael a chance to get used to them. And the reason that He did so specifically with regard to these two sets of Mitzvos - is because Arayos and Ov and Yid'oni (to know the future) are particularly attractive and exceptionally difficult to remove from one's system - See Oznayim la'Torah who elaborates.
Why does the Torah insert the (otherwise superfluous) words "Liznos Achareihem"?