Thanks for the insightful opening for me to offer my variation on that staging theory which I don't think I've repeated here in over a year.
The "ransom" note was most likely written a day or so ahead, and not for the purpose that later developed. The original purpose was probably to create a fake kidnapping as a cover for sexual abuse, presumably by a family member, that PR feared would be visible to the family doctor. Other reasons for the note are possible, perhaps as simple as faking a failed kidnapping that could also be used as an excuse to go back to Georgia.
That precise plan to create a "failed" kidnapping, and the manner in which that "plan for a failed kidnapping" itself failed, are not really knowable except by admission of someone directly involved. But when it failed, resulting in serious injury to JonBenet, the note was already there as a prop. (True, a family member or close friend might have intervened to again abuse the child, inflicting an injury independent of the plan, and the note was used to cover that drastic development.)
The note is the key. It's oddities to me are only explained by a scenario like I have described. There's too much content that no one could come up with in the aftermath of his/her child's death, and it's obviously preposterous for an intruder to leave incriminating evidence behind if indeed there existed an intruder who would or could write such a note in the first place. It is as DeeDee249 suggests, the product of more than one person bandering around ideas and recollections of what a fairly convincing note might sound like, and not very skillfully done at that. But they never expected there to be a very thorough investigation as they expected to be out of Boulder with the "victim" that presumably John saved, as the police ponder that mysterious note, and the family's avoiding all contact "for safety's sake" in Georgia.