Seeing as, at Har Sinai, Moshe had initially maintained that "Erech Apayim" should be confined to Tzadikim, why did he mention it here, where the people were Resha'im?
Rashi and Ramban (citing Sanhedrin, 111a): When Moshe suggested that 'Erech Apayim' is fitting for Tzadikim and that Resha'im should perish, Hashem replied that it applies equally to the Resha'im. Consequently, when Moshe mentioned "Erech Apayim" here, and Hashem reminded him of what he had said at Har Sinai, Moshe countered by reminding Hashem what He had said, and that "Erech Apayim" was perfectly appropriate, both by the Eigel and here.
And how do we reconcile "Dor Shelishi" with "Dor Revi'i"?
Seforno: If the fourth generation sin more that their parents, then Hashem will destroy them immediately; 1 whereas if they sin to the same extent as their parents, then He will wait for the fourth generation before destroying them.
Seforno: Since there is no hope that they will do Teshuvah.
Why did Moshe omit the Midah of 'Emes' and that of 'Notzer Chesed la'Alafim' here?
Ramban: Because according to the Midah of Emes, they ought to have been destroyed; whereas 'Notzer Chesed l'Alafim' is based on Z'chus Avos, and Z'chus Avos was not appropriate here, since the people were rejecting the Avos and the gift (of Eretz Yisrael) which the Avos yearned for. 1
Why did he omit "ve'Chata'ah" and "Rachum ve'Chanun"?
Ramban: "ve'Chata'ah" - because "ve'Chata'ah" has connotations of Shogeg, and Yisrael in this instance were Meizid; "Rachum ve'Chanun'
See also Tosfos (Hadar Zekeinun) as to why Moshe omitted six of the thirteen Midos of Rachamim.
And what are the implications of "Poked Avon Avos al Banim"? Why did Moshe insert it?
Ramban: Knowing that Hashem would not forgive them completely, 1 Moshe asked, that if their sin will not be forgiven, even if their death-sentence is commuted to one of long-term, then their sin should be passed on to their children. 2