That's funny, I always have the same reaction to IDI theory - everything the Rs do is because of stress, or shock, or normal forgetfullness, whereas everything the never ending list of possible intruders did was suspicious.
As to why FW went with JR to the basement - who knows. It doesn't strike me as suspicious. He was called over by PR. He didn't just show up on his own, and wouldn't have had much excuse to be there but for PR ignoring the warnings of the RN. Since he'd already searched the basement, it might make sense for him to look elsewhere, but then again, he was JR's friend, so what is suspicious about following one's friend as he searches for signs of an intruder or signs of anything of JB's being taken?
One question. If you think of FW as a suspect, then presumably you think he was molesting JB and killed her to prevent her from talking. Why then, the blow to the head, the probable manual strangulation, and the garrotting? I mean, it should be relatively easy to kill a six year old w/o doing all that. Just choking her would have been enough. And why would FW leave a fake RN? He could just kill JB, then leave.