And in two sentences I already know all the information that these scientists don’t.

The spirits again for children because they have the freshest organs and the longest life span ahead of them.

The spirits after taking an organ becomes said organ, meaning that using spirit magic bs the child’s body still functions as long as the organ monster is alive.

Because the monster is acting in place of the organ, once the kid dies the organ dies because, think about it. Has a heart ever keep on beating after the body dies? So in it’s monster form, it protects the host because that is where it gets it’s nutrients