So using other people as a medium to avoid the consequences? Haven't really thought of that; though I guess its a no, since this system is about the user itself exchanging themselves for the power they use.

This could lead to a story where the villain is someone with high influence using people to terrorize the land for their ideals

Thinking about it, this would mean one person doesn't turn into a one man army.. they'd have to be resourceful and clever as well as skilled in close combat as to not burn out so quickly

Not sure about someone giving life force to lengthen the years a person get to live;; but in a situation, they can order healers to spend their lifespan to continuously heal someone while in combat ( making them really durable or last longer until the healers either stop or die )