So the fact that we can contact these spirits and ... interact with them, to a certain degree, may be what differentiates us from whatever's outside the barrier? I don't believe we've ever heard anything about the Malnosso being able to contact the spirits.

The spirits aren't violent. And I don't see any reason why the spirits would allow themselves to be used as weapons against other living creatures... I don't recall any of the possessed being aggravated towards the Malnosso.