No its not impossible for an RDI to come up with an working intruder scenario, but I bet it would be hard for the IDI's to come up with an RDI scenario...
I think you're right, Ravyn. I have a minor theory on this (LOL). I think IDI are very one-dimensional in their views of Patsy and John. There's this perception of her as being this perfect, shiny, sweet church lady who just didn't have the temperamental wherewithal to commit this crime and of his being this perfect, dynamic, industrious family man who similarly couldn't harm a fly. RDI may be critical of Patsy but they do at least show her the respect of crediting her with the normal bumps, layers and grooves that we all have and of being a flesh and blood woman who was living with stresses that most us can only imagine.
This isn't for the purposes of bolstering my own RDI convictions. Just an attempt to explain why RDI can envisage PDI/JDI scenarios and accept that there are IDI scenarios that they find much less persuasive while IDI often can't even begin to think of an RDI scenario. Having said that, I know a number of our in-house IDIs are a bit less intransigent on the point.