Whose sword is the Pasuk referring to?
Rashi, Targum Onkelos and Targum Yonasan: The expression 'Hava'as Cherev' always pertains to that of an invading army. 1
Targum Onkelos does not mention an army.
What are the implications of "N'kam B'ris"?
Rashi#1: It implies that there is a vengeance that is not based on the covenant, such as the blinding of the eyes of Tzidkiyahu. 1
Rashi #2, Rashbam Targum Onkelos and Targum Yonasan: It means that Hashem will avenge the B'ris that He made with you and which you abrogated. 2
Which is not hinted in the current Parshah.
Rashbam: See above, Pasuk 15.
Presumably, the Seforno interprets "Sefer ha'Beris" as Mishpatim, which contains the Mitzvos that Hashem gave Yisrael at Har Sinai, where He entered into the B'ris with them.
Seforno: As occurred in the days of Malchei Yisrael, as foretold by the Navi Elisha in Melachim 2, 8:12.
Seforno: See Kesuvos, 30a.
What is thee meaning of "Vene'esaftem"?
Rashi: It means that they will be gathered from the fields outside into the towns, on account of the siege.
And what is the connection between the pestilence and being handed over to the enemy.
Rashi: When they go out to bury the people who died in the plague of pestilence, 1 the enemy will capture them.
Seforno: Being handed over to the enemy refers to the ten tribes falling into the hands of Sancheriv, King of Ashur.
Rashi: Since it is forbidden to leave an unburied corpse in Yerushalayim overnight.