1)
Why does it say "Im Ben Im Bas (singular) Yatzilu"? Above, the plural was used!
1.
Radak: Not many, and not even one will be saved. It is normal for verses to deviate the repetition. Also the plurals are different - "Im Banim v'Im Banos" (16), "Banim u'Vanos" (18).
2.
Malbim: Here, the Chidush is that they will not save even a lone child. Dever infects more when many people are together.
2)
Why does it say "b'Tzidkasam Yatzilu Nafsham"?
1.
Malbim: Here there is need to look at their great virtue to save themselves, for 'once the destroyer is authorized, he does not distinguish between Tzadikim and Resha'im' (Bava Kama 60a).
