Why does the Torah change the Lashon from "ve'Ish asher Yishkav... " to "ve'Ish asher Yikach ... "?
Why is the first of the two women Chayav S'reifah? Did he not marry her be'Heter?
Rashi #1: The Pasuk is speaking where he 'married' first his mother-in-law and then her mother, both of whom are forbidden.
Rashi #2: "Es'hen" means 'one of them'. 1
Rashi: Because 'Hein' is Greek for 'one'.
How is the burning carried out?
Targum Yonasan: By pouring molten lead down the adulterers throat.