Why does it say "u'Vnei (plural) Karmi"? It lists only one!
Refer to 2:8:1:1.
Malbim: Ezra copied his Sefer from the great Sefer of lineage, which had the lineage of every family and its branches, from beginning to end. He copied only what is needed, and special people. The great Sefer listed many Bnei Zimri, and among them Karmi. It listed many Bnei Karmi, and among them Achan. Ezra wrote u'Vnei, like it said there.
Who was Achar ben Karmi?
'Rashi': Surely this is Achan. He is called Achar, for he was Ocher (clouded) Yisrael 1 via transgressing the Cherem. Due to him, 36 men died. According to what I explained, that Ezra tells whatever detriment he can 2 , therefore kingship did not come from them.
Radak: Surely Karmi is ben Zavdi, i.e. Zimri, who was mentioned. This is the custom of this Sefer, when recording lineage. Achar is Achan.
Malbim: Also, Reish and Nun are interchangeable - sometimes Nebuchadnetzar is called Nebuchadretzar. (Usually, interchangeable letters are pronounced from the same place. Nun is pronounced from the tongue, and Reish from the teeth (Maharsha Berachos 6b)! - PF)
Above (1:32), 'Rashi' said so about Bnei Keturah, for the honor of Yitzchak. Does the same apply to Yisraelim?! (PF)
