Why does the Torah change the expression from "ve'Ish asher Yishkav ... " to "ve'Ish asher Yikach ... "?
Rashi: Refer to 18:17:1:1.
Ramban: Refer to 20:17:1:1*.
Bearing in mind that the adulterer married the first of the two women be'Heter, why is she also Chayav Sereifah?
Rashi #1: The Pasuk is speaking where he married his wife, then her mother, and then her mother's mother, in which case the latter two are both forbidden.
Rashi #2: "Es'hen" means (not 'both of them', but) 'one of them'. 1
Rashi: Because 'Hein' is Greek for 'one'.
What are the connotations of "Zimah"
Ibn Ezra: Adultery in the mind.
Targum Onkelos: A scheme of sin.
Targum Yonasan: Adultery:
Why does the Torah insert the (otherwise superfluous) phrase "Zimah Hi"?
Sanhedrin, 75a #1: To teach us a Gezeirah Shavah "Zimah" - from his wife's blood-relatives (here) to "Zimah" - by his own blood relatives, 1 that the adulterer is also Chayav Sereifah.
Sanhedrin 75a #2: To teach us a Gezeirah Shavah from the end of Acharei Mos - Refer to the previous answer - that the seemingly lighter Isur (of Eim Chamiv and Eim Chamoso) receives Sereifah like the basic Isur of Chamoso - just as Bas Bincha and Bas Bitcha are Chayav Sereifah like Bincha and Bitcha.
See end of Acharei Mos, 18:10. Torah Temimah: The Pasuk does not actually write "Zimah" there. However, the Gemara adds an addional D'rashah ("Heinah" by "Ervas bas Bincha O bas Bitcha" - 18:10) from "Heinah" by "Ervas Ishah u'Bitah - 18:17, where "Zimah" is written, to complete the Limud.
What are the inplications of "Oso ve'Es'hen"?
Sanhedrin, 76a: It implies that the Chiyuv Sereifah only applies when both women are alive, but that, having relations with one's mother-in-law after the death of one's wife is not subject to Sereifah. 1
See Torah Temimah, note 34, who discusses at length the Halachah of Chamoso after the death of one's wife
How is the burning carried out?
Targum Yonasan: By pouring molten lead down the sinner's throat.
