It's takes a lot more stretching to make someone else responsible for NC's murder. Say, like JP. First you have to manufacture some motive that claims him as the father of NC's 2nd child, him finding this out, even though NC told him baby K wasn't his, him having a problem, him wanting to kill NC to avoid detection as baby K's father. Mmmmkay.
Then you have to make sure he knows where she is on 7/12/08 by 7am, is able to murder her with no defensive wounds, is able to dispose of evidence of that murder, dumps her body in broad daylight, not have anyone see him...oh...and it really, really helps that BC played along on his side, with all that cleaning, early morning HT visits, throwing away of his own shoes, disposing two mismatched shoes of NC's, guessing correctly the jogging bra NC was found in, lying and lying and lying, doing searches on the family computer that look like ones Nancy would do.
Yep, stretching to the point it could be a new advanced yoga pose.