1)
Why did they mention the Name 'Hashem', which is the Name that denotes Rachamim?
1.
Rashi: Because they accused the Midas ha'Rachamim, of being insensitive to their cries, insinuating that Hashem was being cruel.
2)
Why did Hashem not accept their Tefilos - despite the fact that 'the gates of tears are never shut?
1.
Ramban: Because Hashem had issued a decree with a Shevu'ah, and a decree that is accompanied by a Shevu'ah cannot be repealed. 1
2.
Seforno: Refer to 1:45:3:1.
3.
Refer to Bamidbar, 14:41:3:1-3 and notes.
1
See also Oznayim la'Torah.
3)
Why did Moshe add "ve'Lo He'ezin aleichem"?