BC, I think it's possible there was a 5th person (not an intruder) in the house that night who was involved in JonBenet's death, but I don't agree with your reasons for believing there was. Here's why:
After seeing Patsy's exemplars compared to the handwriting in the note, I'm convinced that Patsy penned the note. No one could mimic every nuance of her writing so perfectly.
One of the three Ramseys could easily have disposed of the tape roll, etc. when they left the house on the 26th. None of them was searched by LE. Also, Auntie Pam could have removed the evidence when she did her sweep of the house.
The so-called "foreign" DNA is likely just "noise," or a false positive resulting from the amplification process.
Patsy's "they" comments probably referred to the small foreign faction in the note.
As other posters have said, it's doubtful the Ramseys would have known about the immunity law for kids under 10, so I think the coverup was to protect Burke. Also, I don't think the Rs would go out on a limb for a friend, or even for another family member, under such horrible circumstances.