Boy, I sure would have loved being a fly on the wall at the Frazee household the day it was announced Kelsey was missing. I have had the feeling all along that there were no warm and fuzzy feelings between KB and SF. I also think that PF was definitely a mama's boy and she most likely had at least an inkling that PF had something to do with the disappearance. That said -- she probably asked him outright. He may or may not have told her the details, but my feeling is that she knew. If mother and son are that close it would be unlikely that she couldn't read the body language.
His motive: my opinion only -- KB may have told PF that she wanted to end the relationship and move near her family. PF wanted the baby to stay local. PF did not want to pay child support. Since he'd possibly made several solicitations for murder, it would appear to me that this had been a sore subject between them for a little while. I think he picked up the baby during the same hour he killed KB. I really don't think she had any kind of "inside information" on any kind of illegal dealings he had going on or anything like that.
My thoughts on why CB spoke of KB and PF's relationship as being solid: I believe it has to do with the investigation and perhaps LE asked her to keep a stiff upper lip so that PF would't be tipped off that they suspected him all along. That he may let his guard down and make some mistakes along the way.
As for them not naming any others in the case, I think that the person or persons PF solicited are being left unnamed until this goes to trial to prevent them from being harassed and/or questions by friends, family, reporters, etc. They've got to keep to the specifics or risk tainting the case. It's still quite possible that that person or persons could face some sort of charges, but that will likely be resolved separately.
