What is the significance of the word "Pigreichem" which appears three time in connection with the punishment of the Dor ha'Midbar?
Oznayim la'Torah: It emphasizes that the entire generation died without burial, 1 as the Midrash relates.
See Oznayim la'Torah DH 'Yiplu Pigreichem', who elaborates at length.
Why did Hashem add the (otherwise superfluous) words "ve'Chol Pekudeichem le'Chol Misparchem"?
Rashi: It is to incorporate whoever was counted from the age of twenty for giving half-Shekalim and serving in the army - thereby precluding the tribe of Levi, who were counted from the age of thirty. 1
See Torah Temimah, note 19.
What is the significance of the fact that those who were over the age of twenty died?
Bava Basra, 121b: It precludes the Levi?im, who were not counted from the age of twenty, from the current decree. 1
See Torah Temimah, note 19, who discussesthe reason for this.
Why does the Torah add the (otherwise superfluous) word "mi'Ben Esrim Shanah va'Ma'alah"?
Bava Basra, 121b: To extrapolate, via a Gezeirah Shavah "va'Ma'alah" "va'Ma'alah" from Erchin (in Bechukosai, Vayikra, 26:37) that the over sixties had the equivalent Din to the under twenties 1 - and that they were therefore not subject to the decree of dying in the desert.
See Torah Temimah, note 20.