What are the connotations of "Lo Yikach Ish ... "?
Rashi: It means that Kidushin with the woman in question is not effective.
Who is "K'naf Aviv" referring to?
Rashi: It is referring to his father's Shomeres Yavam (after the death if his [father's] brother),
Having already taught the Isur of Eishes Aviv in Vayikra 18:8, why does the Torah see fit to repeat it here?
Rashbam: To teach us - by virtue of its juxtaposition to "Lo Yavo Mamzer ... " - that the child born to them is forbidden to marry into the K'hal Hashem.
Targum Yonasan: The Pasuk here is referring (not to his father's wife, but) to a woman whom his father raped or seduced. 1
See Peirush Yonasan.
Having already taught the Isur of Eishes Achi Aviv in Vayikra 18:14, why does the Torah see fit to repeat it here?
Rashi #1: To subject the son to two La'avin in the event that he transgresses.
Rashi #2: To teach us - by virtue of its juxtaposition to "Lo Yavo Mamzer ... " - that 'Ein Mamzer Ela me'Chayvei K'risus' 1 - It is only a child born of Chayvei K'risus8 who is a Mamzer, but not one born of Chayvei La'avin.
Rashi: Kal va'Chomer from Chayvei Misos Beis-Din.