I think the red heart is nothing more that what it is. JB either drew it herself or some other child did. Kids draw on themselves and others all the time, especially girls. I know my 3 daughters and their friends did it all the time.
I think it was done previous to her death and during all the time she was moved, and the autopsy and all the involvement investigating her body for evidence, what appeared at first to be a heart, morphed into something else resembling less than a heart. Just think if you drew a heart on the palm of your hand right now, with all that we do with our hands, this time tomorrow, it wouldn't appear as fresh as it did when first done.
Shrug....just my thoughts...little as they are