I will say, I still don't have her that high on my list of suspects, but the ex-girlfriend of Carnell said a few strange things in the show.
She said that she and Carnell had been talking again for a little while. Then she says definitively that they "WERE going to get back together." As far as I see it, you've either have agreed to get back together or you haven't. Saying you "were" going to get back together when one of the people is dead and can't agree to do so (and apparently hadn't yet agreed at the time of his death) is strange. Maybe it's just an emotional thing and she wants to be seen as more like a widow and not the way people are treating her.
The other thing is that she said she knew nothing of Carnell and Kate's friendship dynamics. I understand that it was also stated that when either was in a relationship, they'd pull back a little bit, presumably because they were now receiving emotional support from the opposite sex from their partners. But it's still odd that she knew nothing of it if they were close friends. People have pointed to the strangeness of Carnell's family having no idea who Kate was. It was a little weird, but not so much given the age at which their friendship began and the geography of Cleveland (Carnell, Kate and Kate's family were all West Side-based, Carnell's family is East Side-based). But his girlfriend not knowing anything about their dynamics is almost weirder, although I assume she knew who Kate was at least.