What is "k'Mahpechas"?
Radak: It is an exaggeration 1 . It is a noun (overturning) Samuch to the one who does it (Hashem). In Devarim 29:22, it is Samuch to the object (Sedom).
Radak citing Targum Yonasan: My word will distance you like Hashem distanced Sedom, Amorah and its neighbors.
I.e. it is not really like the destruction of Sedom, which was via fire from Shamayim. (PF)
What are "Shecheneha"?
Radak: The neighbors of Sedom and of Amorah, i.e. Admah, Tzevoyim and the others 1 .
What are the others? Devarim 29:22 mentions only these four. Rashi (Bereishis 18:24, 19:21) says that Hashem planned to destroy five cities, but He saved one (Tzo'ar) due to Lot's Tefilah! (PF)