1)
What are the connotations of "Pen Tishkach es Hashem"?
2.
Targum Onkelos and Targum Yonasan: It means 'lest you forget the fear of Hashem'.
2)
Having written "Pen Tishkach es Hashem Elokecha", why does the Torah add "Levilti Sh'mor Mitzvosav"?
1.
Oznayim la'Torah: It wouldn't be so bad if they forgot Hashem but still kept the Mitzvos, as the Yerushalmi states in Chagigah, 1:6 'If, after forsaking Me, they would only keep My Mitzvos, the light in them (the Mitzvos) would brng them back!'