G-Snap, great post. I also think that what you described above is a very plausible scenario.
IMO, it's very possible that the parents were aware of the killer, who almost certainly is an adult male, having an unhealthy interest in and/or an inappropriate relationship with, JC. There may have been something that either came to light or occurred that Sunday that brought the situation to a critical head. Analyzing JC's digital footprints along w/ those of Denise and James may provide LE with a lot of helpful information.
Re: Denise appearing to have barricaded herself in the bathroom, a question for the parents here: can any of you think of a scenario where you would lock yourself into a bathroom with a gunman loose in the house without first making sure you had your daughter there with you so you could shield/protect her? If Denise barricaded herself in the bathroom and JC was outside, that is IMO a troubling aspect to this case...because again IMO, if the perp was a total stranger unknown to the family, there is no way Denise would have done that.
I don't see this as having been a random crime at all. JMO.
I think Denise and James were the targeted and intended victims. JMO.
I think Jayme was the motive. JMO.