If I were a jury member, and the prosecution presented the above evidence, I would be persuaded that PR wrote the RN unless the defense can explain it and provide specific counter-handwriting evidence along the lines you suggest.
In otherwords, if I were part of the jury, I would accept the photographic evidence as prima facie evidence PR wrote the RN in her own handwriting, and is thus somehow involved in JB's death, and the burden to prove otherwise would rest on them.
With secondary DNA transfer I feel the reverse, it's up to the prosecutors to take a little 6 y/o girl, have her handshake a boy, "good shedder" if you please, and then touch her sides of her long johns and panties, and see if DNA does indeed show up 98% of the time. This would have to be repeated with variations (i.e handwashing, hand shaking other boys and other people, etc.)
It shouldn't be hard to directly test RDI secondary DNA transfer scenarios. If these scenarios show that secondary transfer scenarios does produce DNA in the 3 locations 1% of the time, and unable to do so 99% of the time, what then? What about 50-50%? 20-80%?