Why does the Pasuk refer to Avodah-Zarah as "D'var Hashem"?
Rashi and Targum Yonasan: Because the warning against Avodah-Zarah they heard directly from Hashem, 1 as opposed to the other Mitzvos, which they heard from Moshe.
See Sifsei Chachamim.
What does "ve'Es Mitzvaso Heifar" come to include?
Rashi: It comes to include someone who worships Avodah-Zarah. 1
Targum Yonasan: It comes to include someone who did not do the B'ris Milah.
See Sifsei Chachamim DH 'D'var Hashem'.
What are the implications of "Hikareis Tikareis"?
Targum Yonasan: It means that he is Chayav Kareis in both this world and the next. 1
Seforno (by implication): The Torah speaks in the vernacular. 2
And what are the implications of "Avonah bah"?
Rashi and Ramban (citing Shevu'os, 13a): It implies that he is only Chayav Kareis if he did not do Teshuvah. 1
Ramban #2: It means simply that 'He will receive his punishment'. 2
Ramban #3 (citing Pirkei Derech Eretz): It means that he will die, together with his sin - that he will continue to suffer after death. 3
Seforno: His sin remains with him until he dies. 4